I found what might be the issue:
if the metar string is MALFORMED (which is not of course depending by GSX), meaning there's an illegal/non-standard metar command in it, GSX will report possible icing condition.
This is not really a bug. In fact, we thought about it and, while you can always decide to *not* use deicing if prompted, there's no way to force-call it if you really want it so, in order to prevent the loss of deicing in case there was an error reading the metar, we decided it was better to be safe than sorry, and allow deicing anyway.
I guess this might be confusing, and could be seen as a GSX bug but, in fact, the real cause is that for some reason, the metar string reported by Simconnect was invalid, so we decided to err on the side of safety, and allow deicing in that case.