VATSIM traffic is shown off their actual FL on the ND. ALTM STD not recognised?

[OELVA]

I've noticed on cruise that VATSIM traffic is shown not on the level they're cruising in their sim.
In both screenshots, you can see the ND showing traffic +5400ft above and +3400ft above, while they should have read +50 and +30 exactly as they were exactly cruising on their filed flightlevels. Vatsim-radar showed them right on their FL and the VATSIM ATC client showed them on the right FL too (asked the controllers in both cases to confirm my observation).
I've been on STD altimeter on both flights.
Changing the altimeter setting (while still having STD active) doesn't change anything in the shown deviation.
At the time of writing, a WZZ A320 crossed 5000ft above while the ND shows +52.
In this case, I changed from STD to 1015 (which was the QNH on an airport right below) and icon showed +51 or +50.

Any chance the TCAS (icons) don't take the switch from local QNH to STD into account?