Thanks Grargar.

I would be interested to know how you could check this info so quickly and accurately!

Both games have a changelog.
In addition, there is GOGdb. The last reported size difference for both games is on March 2019, when they received the cloud save update.

Thanks for the info Grargar, very useful and informative.

Will not be updating my library to re-incorporate these titles as they are obviously different to my backups and not to my liking- I am not interested in the cloud.

They wouldn't appear in your order history, since -- until recently -- the only way to get them was to buy any Bethesda-published game, to which they were treated as a free bonus, not a true part of the product.
I would guess that it works the same for any other case where buying a given game gives you another game (or, often, another version of the same game) as a separate item in your library. For example, if you buy Neverwinter Nights: Enhanced Edition (and don't already own the older edition of the game), I assume it will still only show the Enhanced Edition in the order title & receipt e-mail, even though the purchase will also add Neverwinter Nights Diamond to your library automatically.

Yep, this happened to me too. Obviously not a huge deal but it does make one worry, right or wrong, that it could happen to something they paid for.

Can we re-add them manually or is it safer to wait that GOG fixes the issue ?

Just claim the free versions on the website. The only caution I'd give would be if you're using Galaxy plus cloud saves and are part way through a playthrough. I don't use Galaxy so don't know how this would react.

I am also missing Arena and Daggerfall. Hadn't noticed.
They really should have warned about it >:C

Daggerfall and Arena also missing from my GOG library.

Short term, practical effect? Not much because they are backed up on a seperate drive and can most likely be added back by reclaiming on their store pages. And were free to begin with.

Longer term, psychological (trust) effect? Not good. If two items can vanish from the library, any other can.

Reinforces my decision to have already made backups of offline installers for everything previously purchased.

Just re-add them, mate. If GOG tries to fix the problem themselves it shouldn't foul up if we've already added the new game ID products to our accounts in the meantime.

And besides, back up those small installers; Arena and Daggerfall aren't likely to get updates in the future (well, the near future anyway).

Thanks both. They are back :)
The worst is they were still in my library until I refresh my account with www.gog.com/user/refresh. Then they vanished !

So don't refresh your account If it's not necessary !

It may only appear they are there if you haven't done a refresh.
At some point a refresh will occur anyway from GOG, so better you refresh now and then add the two games back properly, as at least now you are aware enough to check and notice.

Of course, anyone who has added them recently doesn't need to do the refresh.

Not just the two elder scrolls games were removed, but I had a Sherlock Holmes game removed from my library. Which I got through a preorder of chapter one.

Not the first time a preorder bonus has vanished:
@GOG Staff: The artbook of Syberia: The World Before has been removed from the accounts without any reason

Someone had two Metal Gear Solid games disappear:
Both my Metal Gear Solid Games missing from my account!!!

Both of these are confirmed errors on GOG's part, as opposed to the games being bought through a third-party grey market.