Sprint issues in biggest gear (free ride) -2T (most likely) instantly cuts all resistance...

Good day and happy new year to you all

Following issue:
I have a few self created sprint trainings, where I start with 110w for 15min in ERG mode, followed by 10s of free ride.
As I approach the free ride part, I usually shift up to my biggest or second biggest gear and start accelerating.
if I go for the second biggest gear and go full power I normally don't experience any issues. most of the time the flywheel slips a bit during acceleration, but I got used to it...
BUT if I go for the biggest gear, it's almost impossible to sprint full gas. after a few revs the Neo somehow cuts out resistance, which is annoying and pretty dangerous, while pushing 1'400w

As a root cause I tested the chain length, changed the chain and even the cassette -it must be the Tacx...

I'm a track sprinter and I can push above 2'000w -which I never even closely achieved on the Taxc, but thats fine :-)

I'm aware Tacx is not a Wattbike or SRM testing device, but this behaviour seems so weird...