I’ve actually got it sorted – it was my mistake all along.
I had a line break before the acf_form_header() (so the redirect didn’t work – but it saved etc. as usual).
Then in order to fix the redirect issue, I set up my own redirect (which needed an exit below it!) by hooking up acf/sav_post which then obviously would redirect but exit before actually saving. I obviously then forgot that I added this bit of code!
tldr: Silly mistake by me!
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