Thank you Wer. You are, of course, correct.
Not all fields in the MC Library are stored as tags in the files. MC has some capability to recognise files that have been deleted previously and then re-imported, and hence reinstate the Library record including additional fields as it previously existed. But if you delete files from the Library, then change tags in it, and then move the files or rename them, MC can't be sure the re-imported file is the same one as it previously knew about. So it will re-import them as new files. Hence, metadata only stored in the Library and not in file tags will be lost.
What has changed now, so that your workflow no longer works, but used to? It could be many things, as Wer says, such as you started renaming files, or started changing important tags that affect the identification of a file, or you started moving and renaming files. Without detailed analysis, we will never know. If you gain more understanding of MC functionality you may be able to work it out though.
isnt that what it's designed to do? Why have various 'levels' of delete? Sorry but don't quite get the logic.
No, that isn't what it is designed to do. As above, MC has some capability to recognise files it previously knew about, but that works best if the file name, location and tags haven't changed. It is therefore most useful if you accidentally deleted, or simply temporarily wanted to delete, files for some reason.
The three levels of delete descriptions are pretty self-explanatory, I think. With some commentary that might be rewritten as;
1. Just remove this file from the Library for now, because I still want to keep it, and may re-import it later. I'm hiding it from the kids/wife because it's going to be a surprise. Or I don't want to get rid of the file, but I'm going to archive it somewhere.
2. Remove this file from the Library. I don't think I want to keep it, but just in case I change my mind, put it in the Recycle Bin. I may want to recover it from there in the short to medium term... or maybe not.
3. Remove this file from the Library. I don't ever want to see this again!
Deletion before doing external updates is neither required or advised.