Why does the Mail app continue to show inaccurate counts for Flagged and Unread mailboxes across all architectures?

Mail continues to have trouble showing accurate counts for the Flagged and Unread mailboxes. I only use one mail account, which is my iCloud account.

As of today:

iPhone 15 Pro, iOS26.1:

Unread shows 0 (0 is correct)

Flagged shows 11 (0 is correct)

iPad Pro M4, iPadOS 26.1:

Unread shows 0 (0 is correct)

Flagged shows 0 (0 is correct)

M3 MacBook Air, Tahoe 26.1:

Unread shows 1 (0 is correct)

Flagged shows 1 (0 is correct)

I see people posting problems about this going back 20 YEARS, and I’ve personally seen this problem for that long.

Short of full re-installs, reboots don’t work, killing and relaunching the Mail doesn’t work, rebuilding the mailboxes on the Mac doesn’t work. The counts continue to appear almost random.

Why is this so hard?