2 weeks after opening a ticket and no answer from Zwift support, so had to try something.
I moved Zwift to Apple TV instead of iPad, and I placed the Apple TV like 20 cms from the Saris H3: so far, no more dropouts. Will keep testing, in any case it seems that after last iOS update, Zwift needs a shorter range bluetooth connection between the trainer and the Zwift client (saying Zwift because this doesn’t happen with other apps such as Rouvy or RGT).
If you can bring your Zwift client near to the trainer, try it.