Intermittent error 2E90 (mixture control) on E93 N54

I have an annoying intermittent 2E90 (Mixture control bank 1) error which pops up maybe 3x in one day or not even once for weeks.

I don't ever get 2E91, which is the same error for bank 2. I've swapped around injectors, coils, plugs - doesn't change anything. I did change the O2 sensors (they were getting old anyway), but it came back anyway. A smoke test reveals nothing. The intake manifold isn't cracked. All the hoses and everything looks as it should. But I cannot for the life of me work out what is causing this. It doesn't help that it might come 3x in a single afternoon or not come once in weeks of driving. There seems to be no connecting factor. I also don't feel anything, no hiccups, nothing, on the drives when it popped up. I don't get a CEL, so I cannot see the exact moment it happens either.
It causes no noticeable drivability issues. The car starts and runs as it should. MPG didn't tank. If I would not see the code on the scanner, I'd have no idea anything was amiss.

Has anyone had anything similar? I am strongly leaning toward some kind of vacuum leak as I doubt both my old and new O2 sensors have the exact same fault...