OK, before reporting this BUG to technical support I think it is a good idea to hear if other users are facing this problem.
I am doing several tests, but the worrying thing is that in the info, [view note info] in the wrong converted note without attachments, the size of the note is reduced to a few kb, which suggests that the attachment not only does not appear for formatting reasons but is actually deleted.
For the moment, if there are no solutions or time extensions, it is unthinkable that I will be able to recover all my attachments and I will have to keep a legacy version offline on a PC to resolve one note at a time and move the attachments into the reconverted notes. A misfortune.