Same for me - have a look at the post I made here: Vaillant inbuilt monitoring vs MID meters - #40 by Andre_K
The problem is that the Vaillant measured flow temperatures are too high - for me around 1.2 K - while the return temperature measures ok. This translates to a too high deltaT for the internal heat calculation. Depending on what your typical flow-return deltaT is, this can be quite drastic. If you’re typically running a deltaT of 10K then the 1 K offset will be only a 10% overestimate, for my scenario with a deltaT of around 3-4K 'm looking at a 30-40% overestimation. You’ll find your individual offset by looking at flow&return temperatures during a time when the compressor is off (between cycles in warmer weather).
So in summary - nothing you can really do about it to get better values to show in the Vaillant app. If you have access to the ebus data you can calculate an approximate correction factor.