Time:
Since most recent patch

Frequency:
Always

World name:
Fields of Glory, Seal Rock, Borderland Ruins

Character name:
All Characters

Retainer name:
N/A

NPC name:
N/A

Monster name:
N/A

Class/Level:
Any

Party or solo:
Party

In-game time:
Any

Area and coordinates:
All frontline instances

Housing:
N/A

Steps:
1. Receive heavy debuff from taking damage while mounted during frontlines
2. Receive heavy debuff from taking damage while mounted a 2nd time, within 60 seconds of previous step. The duration is reduced
3. Receive heavy debuff from taking damage while mounted a 3rd time, within 60 seconds of previous step. The duration is further reduced and the character is now fully immune to heavy for 60 seconds.

Prior to the patch this was not how heavy from taking damge+mounted worked. Upon being hit by any damaging attack, a 20 second heavy debuff would be applied, which was not subject to diminishing returns.

Players are now able to build immunity to heavy fairly quickly and mount while under attack with impunity, defeating the purpose of the mechanic. There is no mention of this change in the patch notes.