I'm down for relationship discourse if and only if no possibility exists of arriving at any useful conclusion.
"Is it cheating if you're on a break" what is this, a sitcom?
"Is it cheating if you're fucking two different instances of the same time traveller and the earlier one doesn't know" now we're talking.
Maybe it's because I'm used to polyamorous relationships where "cheating" is more about betraying agreed upon/reasonable limits than any single objective standard(*), but... just ask?
If you can't ask current!partner because it would break the timeline etc then ask future!partner: would past you consider this cheating? If they say yes then don't do it, even if it's not cheating per se it's messed up (like... regardless of how you feel about it, if they think it's cheating then why are they willing to betray their past self like that??)
If they say no then either they're telling the truth and you have a good reason to think it's within the bounds of your current relationship, or they're lying in which case you have bigger problems like a partner who's a liar, a traveler from an evil alternate universe, etc.
I guess if they say, like, "Past me would think it was cheating, but they are wrong, because unlike me they have not thought very deeply about the fluid nature of self with regards to the space time continuum, and I consider myself the same person as them modulo some life experience" then you have a dilemma. But then it boils down to whether you think of them as the same person, I guess.
(*)This is in no way meant to imply that polyamorous relationships are better, just that the question of 'cheating' in a polyamorous context is already so messy and convoluted that adding time travel doesn't seem like much of an extra complication.