Technically speaking, both of OP's alternatives are syntactically valid, and in context would mean exactly the same thing.
But idiomatically, native speakers tend to avoid using must in contexts where they mean it must be the case that X did / didn't do Y (as opposed to X must [not] do Y = X is/was required / not permitted to do Y).
The must form was more likely a century or two ago, but it's worth pointing out that even competent native speakers today can be a little uncertain about exactly how this verb works (see Is “must” ever grammatical as a past tense verb? over on ELU). Perhaps partly because of that, we're more likely today to switch the verb to...
You can't have done anything to fix it (or equivalently, ...couldn't have...)
...which has the additional benefit of nullifying the problem of whether or not to use the contracted form (since it would be unusual / stilted to use cannot or can not in this context).
The fact of negation is almost certainly relevant here - most native speakers would agree that in practice, the opposite of It must be true is nearly always It can't be true (not It mustn't be true).