mail often swaps subject heading for messages

The problem just recurred again in one of my mailboxes. It seems to have involved 2 different messages swapping subject headings (if there are other similar problems in other mailboxes I haven't noticed it yet).

So I went to iCloud Mail (using Firefox) and the problem didn't exist there. The message headings appear in the same order in my mailbox, but the two messages had swapped their content. The rule I use to put those messages into this mailbox selects messages that are sent to a specific e-mail address of mine (actually an e-mail alias that I had set up on Mac.com a million years ago). I don't know if that causes problems in Mail or not, but most of the messages in that mailbox did have the correct subject headings. Just those two (as far as I noticed) were swapped.

On my iPhone the messages have the correct subject headings and content. On my Macbook Air the messages are also correctly attached to their subject headings.

Thanks for your reply, MoonJ.

I have kept my computer up to date as far as Catalina goes, but because of software compatibility issues, I don't want to upgrade to Big Sur until I'm certain it won't render any of my software obsolete.

While the problem is recurring, it is also intermittent enough that I don't want to run my computer in safe mode all the time just to see if it ever happens again. I'm concerned safe mode would interfere with my daily work habits and routines.

I suspect this is not a login item issue because I have (on occasion) seen it happen on my Macbook air, too. That would seem to mean that it's not related to a user account on my iMac as well. (I read mail on my Macbook much less often than I do on my iMac so I'm not sure if the problem is as common there or not)

I have read other threads from years past in which people had this same issue, though I didn't see where anybody said it is an intermittently recurring issue. I'm wondering if maybe Mail sometimes gets confused because I'm collecting messages from four different servers and using rules to distribute it all into many different mailboxes in my iCloud account? Or is something wrong with my iCloud account? (I used to put all my mailboxes "On My Mac", but started putting them in iCloud after I got a new iMac a few months ago. I only started having this problem after I did that.)