GSX reads the metar from Simconnect itself, it cannot read "just" the temperature, it must read the metar, because is the only way to read the Dew point.
This Simconnect metar, doesn't have anything to do with an external metar you might have seen reported either online or from a weather engine. It's an internal metar report, which is created by the simulator, so any addon needing to read weather, will get in in a metar format.
So, looking again at the code (and I think we made it intentionally), if the metar can't be read, either because it's not working, sometimes Simconnect gets stuck and doesn't report metar in time, or because the string is illegal/malformed/nonstandard, the fallback was to ALLOW deicing anyway.
That's for the reason I already explained: you CAN refuse a deicer if icing conditions has bee mistakely detected (not because GSX's fault, but because it got a bad Metar from the sim), but you cannot force-call it if no icing conditions to begin with.
So, assume an airplane like the FS Labs, were icing can be DANGEROUS on the flight and, for any reason the metar couldn't be read, what GSX should do then ? Not allowing icing, because it cannot be *sure* of the temperature/dewpoint, or allowing it anyway, since you can always refuse the service ?
I ASSUMED this was the safest choice but, of course, if it's mistakenly identified as a GSX "bug", maybe it's best to change the code, and block deicing in case of a bad metar report.
The "fix" (which is a downgrade IMHO), is already online in the current Live Update so, you can try it now.