But mine (5B1F0DAF378A1FAFCFD5FA9CDC66D1023DC0276E) lost guard status
as well in december, and I have no idea as to why either - except
possibly because of running a non-recommended tor version (i.e. too
bleeding edge release branch state). That is fixed since two weeks or so,
and the addresses never changed.
No, the tor version is not used to determine the guard flag.
It's a combination of stability, speed, and age:
Your relay may have been unstable with the extra client load in December.
Or its bandwidth might have dropped.
T