Employment contracts for non-executive positions in the US are pretty rare, but hey.
(My advice is only valid in the US, where at-will employment is the standard)
If it is just an offer letter, I would say that you should mention (email is best because it's written) to your recruiting contact and your hiring manager that you'd discussed hybrid-remote working and just wanted to make sure you understood expectations properly, since they weren't in the letter, and then when they reiterate your understanding, thank them and sign (if that's your last pending concern).
There is basically no incentive for them to hire you on that kind of false pretenses, because you'll be angry and if you don't suck you'll probably end up quitting early over it, and if they want you gone, you're at-will anyway.
If you have an actual contract, then you have a contract and get it in writing or accept that they could change their mind and use it as an excuse to end the contract early if they chose.
I am in the EU and talked and it is a small company. The CEO mentioned that some colleagues spent a lot or time in other EU countries so it seems that it is not uncommon. However there is nothing mentioned in the contract. Then again, it is a small company and things might be a bit more informal.