Rift S failing to track head position?

Got to use my brother's Rift S a bit recently. When I first tried it a few weeks ago, everything went perfectly well. Got to try it again yesterday, and I got a weird issue - head movements were tracked normally, so I could look up, down or sideways without problems, but it didn't seem to notice any changes of head position (e.g. leaning slightly forward to get a better view of something), it felt as if the whole image was moving with me, instead of me being in the situation.
It also happened in Oculus VR itself, so it wasn't only an in-game problem. (Actually it started out correctly on Oculus, but after a few moments it suddenly "switched" to this issue.

Tried it again today, same problem.

Has anyone else had this problem, and/or knows how to fix it?
 
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