According to Dow (I can’t find the e-mail now)
Propylene glycol antifreezes are designed to provide burst protection to temperatures of -50° F…
Ice crystals will start to form in -50 RV Antifreeze at temperatures around +10° F, and it will appear to be solid ice at around -10° F to -15° F.
Propylene glycol based antifreezes continue to contract and will not expand until temperatures of -50° F are reached, thus providing burst protection for pipes…
Call antifreeze mfg. and report it and ask, if in doubt…