In that case you should have had a response by now, but some tickets seem to get lost somehow.
There appears to be only 2 of you with the problem both with Simworld OH and FDS XPNDR's so there is a screw up in the config there I would think, but if Prosim cannot see where the problem is from the info you have given them, then what else can they do?
I have a Simworld O/H but it was built by me, not assembled and OC XPNDR and I have never had the problem. I have had plenty of others though but they were usually down to me or a dodgy I/O card.
Is it really a big deal if the squawk code goes to 0000 when you switch from GND to APU power. I usually set mine just before taxi when the engines are running. Its part of my checklist.