Yes, I had this with a customer too..
I created a temp directory
move all mail from inbox to temp dir
let apple mail sync
move some mails back to inbox
let apple mail sync
move some mails back to inbox
let apple mail sync
move some mails back to inbox
let apple mail sync
etc.
finally all mail was downloaded.
last week the customer called again with same problem and customer solved it same way.
Last weekend I upgraded to v16, don't know status of problem with customer now.
in my opinion it's the unliable apple mail software.
By reporting abuse, a moderator will be asked to review the content of this interaction to take further action, if necessary.