This is intentional, performer must be a username with no prefix for *all* log types, not just thanks.

I should have probably explained this better :)
In English, there's the Performer and the Target (title or user) field.
In Italian, the translation does not mention the (title or user) bit, so maybe that's a first source of confusion here. But even if we changed the translation, that wouldn't reflect the fact that in the Thanks log (unlike what happens in other logs), whatever is typed in the Target field gets the User: bit added. So it's not just that it simply doesn't work when you copy/paste the same string in the previous field, but also that it feels weird to see two adjacent fields which behave so differently.

(In reply to Elitre from comment #2)

But even if we changed the
translation, that wouldn't reflect the fact that in the Thanks log (unlike
what happens in other logs), whatever is typed in the Target field gets the
User: bit added.

This actually is consistent with other logs. Compare:

• https://en.wikipedia.org/w/index.php?title=Special%3ALog&type=block&user=Superm401 (block log, unprefixed performer)
• https://en.wikipedia.org/w/index.php?title=Special%3ALog&type=block&user=User:Superm401 (block log, prefixed performer, User: is treated as part of the username, so it doesn't work as intended)

• https://en.wikipedia.org/w/index.php?title=Special%3ALog&type=block&page=Willy+on+Wheels (block log, unprefixed target, auto-prefixed)
• https://en.wikipedia.org/w/index.php?title=Special%3ALog&type=block&page=User:Willy+on+Wheels (block log, prefixed target)

So unless there is some difference I'm missing, this is a general issue with Special:Log. However, it does seem fixable (or enhanceable), so moving components.

It works the same way for All the logs. There should be a message explaining that, above the text-fields. eg. at [[Special:Log]]...

Oh, I see the message at [[mw:Special:Log]] is totally unclear, so I guess the default (and therefor widely translated) message doesn't explain it. Gah! But it needs to work for both mainspace pages and elsewhere
e.g.
https://en.wikipedia.org/w/index.php?title=Special%3ALog&hide_thanks_log=0&page=user%3AJohn
https://en.wikipedia.org/w/index.php?title=Special%3ALog&hide_thanks_log=0&page=John

[rewrite #4, edit-conflict #3...]
Ah, I now see the difference, when we use the drop-down selector for just the "Thanks log".
This is all related to bug 46494, which was originally about clarifying the confusing documentation message, but was closed following a pointer to the code that makes *user-specific-logs* automatically-add the "user:" prefix. Eg.
https://en.wikipedia.org/w/index.php?title=Special%3ALog&type=thanks&user=&page=John
will automatically add "user:" prefix, as will core's "block", "newusers", and "rights" logs, and possibly other extension logs using the code-hook.

Given that the code is trying to be helpful, and going back to the old way would be equally confusing, I'd suggest that just updating the default interface messages, would be best.
I.e. Put the clearer explanation from Enwiki's [[MediaWiki:Alllogstext]] as the default.
Possibly altering [[MediaWiki:Log-description-thanks]] would also help?
(Sidenote: I've asked, and to change default English messages, we have to file a bug here. English root messages don't get rewritten at translatewiki.)

(In reply to Quiddity from comment #4)

Given that the code is trying to be helpful, and going back to the old way
would be equally confusing, I'd suggest that just updating the default
interface messages, would be best.

Your example shows that the target (right-most field, which corresponds to page in the URL) is auto-prefixed. I think people generally support this behavior.

I think the request is to do the same (auto-prefix) for the performer field (left-most field). I don't believe that is the old way.

(In reply to Matthew Flaschen from comment #5)

I think the request is to do the same (auto-prefix) for the performer field
(left-most field). I don't believe that is the old way.

Sorry, that's not right. The idea is to allow the performer prefixed (User:Foo) or unprefixed (Foo). Currently it has to be unprefixed.