By the way, this also applies for "unforeseen problems" which "made the project go behind schedule" and "now we're going to work extra hard" (meaning overtime without pay).

It's up to management to not only account and prepare for Known Unknowns (problems known to happen but not if, when and how severe - for example employee sickness), but even have margin for Unknown Unknowns (problems that nobody expected).

In fact, half way competent managers will do enough analysis and research upfront to transform many otherwise Know Unknowns into Known Knowns (we know this will happen) and Unknown Unknowns into Known Unknowns - sometimes things are only "totally unexpected" because the necessary upfront research and preparation hasn't been done.

Managing this is literally the core job of low-level and mid-level managers, so if they try and dump on you the responsibility for it or try and extract from you out of contract work to make up for it, they are literally acting in the most selfish personal upside maximizing way possible and just covering their own incompetence.

It is not up to you to make sure an incompetent asshole gets a bonus for doing a shit job and overpromising.

This is literally they kind of situation where, unless you're for example reciprocating equivalent leeway from said manager towards you in the past (say, the kind that'll actually quietly let you go home early if you're having a bad day), you absolutely have the moral high ground to leave it to them to sort it out with whomever THEY are accountable to (be it a client they made deadline promises to based on a plan that would only ever work in the perfect zero-problems situation or their own manager).