It shows up 582 times in my latest.log, and
latest.log

This is using endergetic-1.16.4-3.0.0 on Forge 36.1.4

This isn't necessarily an issue we can fix.
The Received balloons for unknown entity is a log warning given when the client can't find the entity a specific set of balloons is supposed to be attached to.
If the warning is recurring many times, this implies a deeper desyncing issue with entity ids.
Vanilla has a similar log warning for passengers that says Received passengers for unknown entity.