From testing done in the Fight Club (warrior) discord when multiple Thunderfurys are attacking the same target and the debuffs changes ownership the TF debuffs appear to momentarily occupy additional debuff slots (instead of two) pushing off valuable other debuffs.
This is not unique to Thunderfury (but this is certainly one of the harder ones to deal with aside from unequipping additional TFs your guild is lucky enough to get) and over in the Warlock discord a lot of work has been done into “phantom debuffs” with the working theory being: On reapplication (of unique debuffs) the server “makes space” for the new debuff before cleaning up the old identical debuff from a different player.
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This also is applied for Buffs. This is why people still see their wboss fly off in some cases.
this might be a unfortunate side effect of batching
the worst thing blizzard has ever done.
remove batching please
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I’m sure blizzard will get right on that. /s It’s pretty sad how many game breaking bugs go completely unaddressed by this company. THIS IS YOUR IP BLIZZARD. At least pretend you care about your property and your customers: start fixing bugs!
Just like HoJ procs getting batched 
do we know if it worked this way in real vanilla? I dont ever remember seeing a problem like that but im fuzzy on a some vanilla stuff now days its been a while.
I remember things being really consistent when strictly controlled. This is that control with a random element tossed in.
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Bumping for eventual Blizz response