What's the weather setup in the multiplayer missions where this is happening? Specifically, the temperature?
If the temperature is significantly colder than standard, it would be expected that the barometric altimeter would read higher than actual altitude, and this is not corrected for by the altimeter setting. At least that is what happens in the real world, so I'm wondering if that is modelled in the sim too.
In which case perhaps the TAD info is driven by the altitude from the EGI rather than the barometric altimeter, and so it's not affected by the temperature? Just a theory that could use some testing as I have seen similar mismatch between baro alt and waypoint alt in missions I've design with cold weather.