TB move some msgs from Inbox to Junk after opening

[thoste]

Since a few weeks I observed I strange behavior of Thunderbird v91.10.0 64bit on Windows 10.

When I receive e new messages in my POP3 Inbox and open it (by double click) some hours later then I can see it at first "as normal".
I close the message window and plan to move it to a message sub directory. in my account.

But immediately after closing I am surprised: The message is gone. It disappeared in Inbox.

Further investigations showed that Thunderbird moved the message silently to "Junk" folder of the same account.

TB seems to inspect a message (again?) when user opens it.
If Thunderbird thinks it is Spam it moves it.

I opened it in Junk folder click "Not Junk" and was able to move it to another message folder.

The messages themselves look not suspicious.
It is totally unclear why TB thinks it is Junk.
It is account independent and happened not in the past (e.g. last year).

However this procedure is somehow buggy
Thunderbird should check for Spam only once BEFORE putting it in Inbox and then never again.

I cannot figure out or even reproduce these scenarios. But it happened approx 10 times now (in 2 months) out of 500 messages.

Does someone else experienced a similar problem?

[Grumpus]

Instead of opening things directly, why not change your settings to fetch headers only.
If you suspect the email label it as junk or just delete it.
Make sure you delete it from trash as well.
It helps to set the junk filtering also.

[tanstaafl]

The order of processing is:

Message filters
"Trust header"
"Adaptive junk" (junk mail controls)

so that seems to rule out a message header. Sometimes (not clear why) messages in a corrupt inbox folder get moved to the junk folder. Try moving many of the messages in the inbox to other folders and then compacting the folder. Don't compact it first, that can make it worse.