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As James says, Thomsons do seem to have a rep for being undersized.
I don’t think I’ve experienced it personally but I’ve only owned one or two Thomson posts and i for sure know a couple people who have had issues.
The fact the post has worked in other frames doesn’t really prove it’s not undersized, those frames have just been a better match for the post within the acceptable range of ‘tolerance’ or the clamp has be more able to take up the slack.
Some manufacturers tolerances are shockingly wide. If you get a post at the narrower end of the range and a frame at the wider then you’re in for some struggle to make it work.
I would put money on a non Thomson post staying put.
Measure the Thomson. My experience is they can be under sized and prone to slipping.