It is possible that the conditions for this bug are as stated by @Lekaa. It makes sense that this bug only occurs with "old" F/A-18C and F-16C slots since there has been a datalink rework lately. All of the servers I fly on, are dynamic campaigns which means that its likely that all slots are from older .miz files (1 year <). This would also explain why it cannot be replicated on SP. (new slots)
I think the solution is to somehow force all F/A-18 and F-16C client slots to use the new version in older .miz files.
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Rissala's post in In multiplayer servers with DL donors, ownship is displayed as a seperate DL track. was marked as the answer