Unsollicited style change of paragraphs

I have not yet been able to identify a precise set of circumstances, but lately (I noticed right after the last update) I have paragraphs which, randomly or so it seems, change from a style to another.

This is somewhat problematic ; – for reasons so obvious I won’t even bother to list them.

I will add to this post if I ever notice a redundancy of factors to this undesirable behavior.
But I have been dealing with this for a while now, and I am still without a clue.

Without much I’m posting anyway, as maybe someone else noticed, and perhaps has a better understanding of what is going on.

All that I can currently bring for a precision is that when a paragraph changes by itself from one style assignation to another, it keeps the formatting of the first, while being assigned (and compiling as) the second.
The only way it can be noticed in the editor itself, is if the original style had a box, while the new style doesn’t. The paragraph looks exactly like before, minus the box.
(I suppose the other way around would be true as well (no box to box), but it hasn’t happened to me yet.)

P.S. I am talking about paragraphs styles.
Nothing to report regarding character attributes styles.