Frequency: Everytime I get down from the mounts for tribe quests or duties
World name: Jenova
Character name: Nui Harime
Class/level: Summoner or Scholor / both Lv.60
Party or solo: solo
Race: Miqo'te Female
Area and coordinates: The Dravanian Foreland (For Vath) / The Sea of Clouds (For Vanu) / Alexander - The Fist of Son (Normal and Savage) / Castrum Meridianum / The Praetorium

Steps (I use Vath as example):
1. This is my original Hotbar 1:


2. I speaked with Vath Kintamer to ride on the Vath mount for completing their tribe quests, and my Hotbar 1 became:


3. After I clicked dismount button, my pet's hotbar showed up and I have to roll it back to my orignial hotbar 1:


Addition info:
1. This starts from patch 3.3, nothing changed in settings.

2. This only happens when a pet is activated. I tested it on both Summoner and Scholor and confirmed.

3. Not only for the mounts from Vath & Vanu's tribe quests. Bird form in Midas A5 or A5S / Cannons in Castrum Meridianum / Magitek Armor in Praetorium will also trigger this BUG.

4. Own mounts with special action such Magiteck Armor don't have this issue.