Answers to Is it bad if hydrazine freezes on a spacecraft? Is it always kept as liquid, or can it be safely allowed to freeze and then thawed when needed? explain that while the freezing of hydrazine on a spacecraft isn't necessarily catastrophic in all cases, it's "bad" because the contraction pulls in more liquid, then the expansion upon thawing can cause damage.
This answer to Is the EU really banning “toxic propellants” in 2020? How is that going to work? mentions:
NASA is testing hydroxylammonium nitrate in space which is expected to perform 50% better than standard propellants and can be allowed to freeze (hydrazine must be kept liquid). LMP-103S has been tested gets about 30%.
Question: what is it about hydroxylammonium nitrate that would make it "less bad" to freeze in space than hydrazine?