Some new gmail goes to ALL MAIL but not INBOX only when I grant permission to Apple mail clients post Catalina upgrade

It is one of the oddities of using Gmail.

Gmail isn't IMAP, but it pretends to be. This doesn't always work out well since IMAP expects a message to exist only in a single folder.

In Gmail, a message can be tagged with many Labels. To pretend it is IMAP, Labels are mapped to folders in the IMAP email client.

When a message arrives to the Gmail server, it is tagged with both an "Inbox" label and an "All Mail" label. If it was marked with High Importance, it would get an "Important" label, also. So, that single message would have to appear in at least two different IMAP folders. When you move a message, or delete it, the Inbox label is removed, and the destination label is added, but the All Mail label remains. Trying to keep track of all of that hocus-pocus would require a dedicated program to handle the Gmail goofiness.

You can disable showing All Mail (and other labels) in IMAP clients. This is probably the safest way to run in a dedicated IMAP email client. In Gmail settings, Labels tab, you can uncheck the box to "Show in IMAP"

If you can find a dedicated Gmail client that also supports other IMAP accounts, that might be a better option. I don't know of any. I access my Gmail in Spark, but my Gmail account is just a giant spam filter. I only use my Gmail account for signing up for lists, like grocery stores, fast food, etc. I pretty much ignore its existence, so that setup works for me.

Catalina Mail does handle Gmail differently than before.

In early versions, it didn't show All Mail at all.

Somewhere around High Sierra, it started mapping All Mail to the "Archive" folder.

From what I've seen posted, it appears Catalina might just map All Mail to All Mail instead of Archive, but I don't know for sure as I stopped using Gmail with Mail.