DoT is Invalid for Leviathan when both head and tail are inflicted

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Date & Time: Aug 18, 2021 12:00pm
Frequency: always
World name: midgardsormr
Character name: Yi Laguan
Retainer name: None
NPC name: None
Monster name: Leviathan
Class/Level: WHM/80
Party or solo: Party, 8
In-game time: Aug 18, 2021 12:00pm
Area and coordinates: The Whorleater Hard, extreme and unreal
Housing: None
Steps:
1. When both head and tail of Leviathan are inflicted with DoT, the damage is completely nullified.
2. The easiest way to reproduce this bug is go to leviathan hard or extreme unsynced with any job using DoTs. For example, if you use Dia of WHM on both head and tail and do nothing else, then leviathan takes zero damage from this. More precisely, it takes some damage from a tick of DoT first and then restore the same amount of HP immediately.
3. This effect extends to all kinds of DoTs. Let's say we have Dia, Biolysis, Bio III, Miasma III, Caustic Bite, Stormbite infliced on head and Sonic Break on tail. Then this bug still happens. This also extends to ground-effect DoTs from DRK, SMN and NIN.
4. Two special cases are thunderclouds from BLM and wild fire from MCH. Thundercloud deals zero damage, maybe because its damage is calculated as DoT. And when a wild fire effect detonates, it also deals zero damage.
5. A demo of this bug can be seen here. https://b23.tv/AafAcc This video is captioned in Chinese because we first find this in CN server, and then we find that there's the same bug here.
*Add more steps if necessary

Connection Specs
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■ PC Specs

To obtain the system information, open the FFXIV launcher and select Config → System Information → Display.

-=-=-=-=- System Information -=-=-=-=-
-=-=-=-=-=-=- System Information -=-=-=-=-=-=-

SYSTEM_OS Windows 10 专业版 64 位 (6.2，内部版本 9200) (19041.vb_release.191206-1406)
SYSTEM_CPU AMD Ryzen 5 2600 Six-Core Processor (12 CPUs), ~3.4GHz
SYSTEM_CORE_NUM SYSTEM_CORE_LOGIC 12 SYSTEM_CORE_PHYSICS 6
SYSTEM_BIOS A.10
SYSTEM_LANGUAGE 中文(简体) (区域设置: 中文(简体))
SYSTEM_MEMORY 24527.125MB
SYSTEM_VIRTUAL_MEMORY 134217727.875MB
SYSTEM_PAGE_FILE 32207.125MB
SYSTEM_STORAGE C:\ SYSTEM_HARDDISK_DRIVE SYSTEM_TOTALDISC_SPACE 417.911GB SYSTEM_FREEDISC_SPACE 52.236GB
SYSTEM_STORAGE D:\ SYSTEM_HARDDISK_DRIVE SYSTEM_TOTALDISC_SPACE 3725.989GB SYSTEM_FREEDISC_SPACE 2323.798GB
SYSTEM_DIRECTX_VERSION DirectX 12
SYSTEM_GRAPHICS NVIDIA GeForce GTX 1060 6GB
SYSTEM_GRAPHICS_DEVICE \\.\DISPLAY1
SYSTEM_GRAPHICS_CHIP NVIDIA GeForce GTX 1060 6GB
SYSTEM_GRAPHICS_MAKER NVIDIA
SYSTEM_GRAPHICS_VRAM 6052.000 MB
SYSTEM_GRAPHICS_SHARED_VRAM 12263.563 MB
SYSTEM_GRAPHICS_DAC Integrated RAMDAC
SYSTEM_GRAPHICS_DISPLAY_MODE 1920 x 1080 (32 bit) (60Hz)
SYSTEM_GRAPHICS_DRIVER C:\WINDOWS\System32\DriverStore\FileRepository\nv_dispi.inf_amd64_b2801df14ec7de03\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nv_dispi.inf_amd64_b2801df14ec7de03\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nv_dispi.inf_amd64_b2801df14ec7de03\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nv_dispi.inf_amd64_b2801df14ec7de03\nvldumdx.dll
SYSTEM_GRAPHICS_DRIVER_VERSION 27.21.0014.6677
SYSTEM_GRAPHICS_DRIVER_DATE 6/7/2021 20:00:00
SYSTEM_GRAPHICS_DRIVER_LANGUAGE 英语(美国)
3.0
SYSTEM_GRAPHICS_PIXEL_SHADER 3.0
SYSTEM_GRAPHICS_VERTEX_TEXTURE SYSTEM_SUPPORT
SYSTEM_SOUND 扬声器 (Realtek High Definition Audio)
SYSTEM_SOUND_DEVICE_ID {A92F3135-466F-4123-8468-0D6210903085}
SYSTEM_SOUND_DEVICE_NAME RTKVHD64.sys
SYSTEM_SOUND_DRIVER_VERSION 6.0.1.8531
SYSTEM_SOUND_DRIVER_LANGUAGE 英语(美国)
SYSTEM_SOUND_DRIVER_DATE 9/3/2018 20:00:00
SYSTEM_SOUND 耳机 (Senngore-S1000 Stereo)
SYSTEM_SOUND_DEVICE_ID {E5E53351-F195-4969-B4A4-FB210A552432}
SYSTEM_SOUND_DEVICE_NAME BthA2dp.sys
SYSTEM_SOUND_DRIVER_VERSION 10.0.19041.1
SYSTEM_SOUND_DRIVER_LANGUAGE 英语(美国)
SYSTEM_SOUND_DRIVER_DATE 12/5/2019 20:00:00
SYSTEM_INPUT {6F1D2B60-D5A0-11CF-BFC7444553540000}
SYSTEM_INPUT_NAME 鼠标
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 3 SYSTEM_GAMEPAD_BUTTON 5 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {6F1D2B61-D5A0-11CF-BFC7444553540000}
SYSTEM_INPUT_NAME 键盘
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 128 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {CA922B20-9756-11EA-8001444553540000}
SYSTEM_INPUT_NAME Controller (Xbox One For Windows)
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 5 SYSTEM_GAMEPAD_BUTTON 16 SYSTEM_GAMEPAD_FOV 1
SYSTEM_INPUT {B169BC10-CD82-11E9-8001444553540000}
SYSTEM_INPUT_NAME Game Mouse
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 24 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {20BD9880-CF36-11E8-8003444553540000}
SYSTEM_INPUT_NAME GIGABYTE Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 3 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {20BDBF90-CF36-11E8-8004444553540000}
SYSTEM_INPUT_NAME GIGABYTE Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 768 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {B16A3140-CD82-11E9-8002444553540000}
SYSTEM_INPUT_NAME Game Mouse
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 0 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {20BDBF90-CF36-11E8-8005444553540000}
SYSTEM_INPUT_NAME GIGABYTE Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 0 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {B16A5850-CD82-11E9-8003444553540000}
SYSTEM_INPUT_NAME Game Mouse
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 0 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {B16A7F60-CD82-11E9-8004444553540000}
SYSTEM_INPUT_NAME Game Mouse
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 0 SYSTEM_GAMEPAD_FOV 0