Saving 1 vocabulary deletes all terms of all vocabualries in the site | Data mining Since I migrated D7 to D8 I suffer from the following problem and need help regarding mining the data that could be useful to make a fine report to the Drupal community.

The story:

When you visit nodes created before migration, you can see their taxonomy terms.

If I save a node with taxonomy data that was created before migration - All its taxonomy data is vanished if its from selection box or radio buttons (if its from autocomplete, the node saves just fine, as weird as it sounds)... It happens in all nodes, and in all content types, (even new ones I create after migration whether from migrated or new content types).

A milestone in my efforts to understand why it happens is this:

When I save any taxonomy vocabulary like in the following image, all of its terms, including the terms of all other vocabularies are just suddenly deleted... Insane as it sounds --- Right after I hit "Save", all of the terms in all vocabularies in the site, just vanish.

But this deletion is only in the UI level; that is, If I go to the table containing taxonomy terms, that is taxonomy_term_field_data I can see all 30 terms that appear in all nodes.

I have consulted a Drupal programmer on this and he told me this is most certainly a migration bug, given it happens on an all-core site and there are no errors in drush ws --full.

Note:

I don't ask anyone to debug this for me; Since I learn basic programming and haven't yet learned to debug, all I am asking is what PDO data should I monitor and present in my report to the Drupal community.

I would thank you dearly for giving me any direction because I suffer from this problem very much.
You should look into the database if the taxonomy terms are deleted indeed or just have changed the tid, the language or anything else.

The most probable is the terms are not deleted but when you try to load them by some parameters the parameters has changed and the result is no terms.