After having read this answer I wondered what happens if hydrazine freezes.
It looks like it is "normal" and not like water, in the sense that it contracts when freezing, unlike water which expands and can cause mechanical damage.
Sometimes hydrazine thrusters are used after several years, and an extreme example would be the reboot of ISEE-3/ICE. But that was circa 1 AU. Sometimes hydrazine is used in spacecraft much farther away; Voyager 1 Fires Up Thrusters After 37 Years, and I am not sure if it could have keep its hydrazine above the melting point all this time, but I don't know.
Per answers there, it seems that Voyager's hydrazine might be kept above freezing, but that doesn't necessarily mean that freezing of hydrazine in general would be catastrophic, but only that it's easier to keep it liquid than to set up some special system to thaw it when needed.