At the beginning of the lockdown, as I didn't have a dedicated computer desk and chair, I was working at my dinner table, sitting on a regular dinning chair. It lasted for about 1 month or so until I broke both the chair and my back (I could not move one morning and had to spend a few days in bed).

It could be you have better sitting discipline (I tend to slouch when I sit), might be that you are more active (I didn't notice any issues last summer, when I was sailing every weekend, but then, I was not sitting at my dinner table for that long either), might be your regular chair is not that regular :)