WordPress updates the database when it upgrades, so it entirely possible that the database is profoundly changed by an upgrade.

I dump my WordPress database daily and back it up (I also use rsync to keep an updated collection of media (mostly photos) in the wp-content folder, because I hate losing or reconstructing my blogs.

I don't have any insight into the issue mentioned by OP though.

On Mon., Nov. 29, 2021, 13:54 D. Hugh Redelmeier via talk, <talk@gtalug.org> wrote:
| From: Stewart C. Russell via talk <talk@gtalug.org>

| On 2021-11-27 18:04, D. Hugh Redelmeier wrote:
| >
| > Do you have shell access?  I think you imply "yes".
|
| Yes, I do, but not to the database server. All I have for that is socket
| access and PHPMyAdmin (blecch).

Ahh.  Kind of "no".

| > Does "fix it" mean "changed the raw data" or mangle the data somewhere
| > downstream of the disk files?
|
| "fix it" meant "broke it".

I quoted "fix it" because I understood that.

| The MySQL DB tables seem to have been quietly
| reprocessed from one encoding to another.

That seems (1) odd and (2) rude.

Is there a chance that the problem is actually in presenting the data
(due to some incorrect setting of a locale somewhere)?

Can you ask you supplier just what happened, why, and if they can
reverse it for you?

There is a chance that the transformation was bijective (or at least
injective) and thus reversible.
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