I remember dimly (I have no personal interest in having a separate /tmp) seeing a poster having exactly this problem a few weeks (or months ? time fly…) ago. IIRC the key was that when one deletes an open file on Linux, it can be kept open for some time, and while its size is added to the ‘free’ space, in fact it is not yet available leading to perplexing results.
Hmm, I did not find it (did not search very hard) but there is a reference a long time ago already: