Message attachments, relocated with Tbird in Dovecot maildir store, not openable; reversible by moving BACK to inbox?

[PGNet Dev]

I run

	dovecot --version
		2.3.17.1 (476cd46418)

on

	lsb_release -rd
		Description:    Fedora release 35 (Thirty Five)
		Release:        35

mail store is maildir

	mail_location = maildir:/data/vmail/%d/%n/Maildir:CONTROL=/data/vmail/%d/%n/_control:INDEX=/var/vmail-index/%d/%n:LAYOUT=fs:UTF-8

front-end client is

	thunderbird --version
	 Thunderbird 91.7.0

also on

	lsb_release -rd
		Description:    Fedora release 35 (Thirty Five)
		Release:        35

mail is delivered to the store via lmtp transport from a Postfix agent

mail, with attachments (e.g., .PDF) is received OK
it's readable, attachments can be opened and/or saved to disk\

if, in TBird client, i move the message to any other folder in the IMAP hierarchy, the message relocates, as expected.

the message itself is still readable, and _appears_ to have the attachment.

but, attempting to OPEN the *attachment*, with e.g. Okular, from within the relocated message pops up a dialog,

	"Could not open file:///tmp/pid-28435/manual_13542AE.pdf"

and saving it to Desktop,

	ls -al manual_13542AE.pdf
		-rw-r--r--   1 pgnd pgnd   27 Apr  1 06:01  manual_13542AE.pdf

	file manual_13542AE.pdf
		manual_13542AE.pdf: data
  
then opening it similarly returns

	"Could not open file:///home/pgnd/Desktop/manual_13542AE.pdf."

Moving the message, via TBird, **BACK** to the Inbox, cures the problem --
-- I can immediately, again, open & save the attachments.

This 'feels' like cache management misconfiguration to me.  Not sure whether in Tbird &/or Dovecot, or what to look for specificially.

Hints as to cause/fix of this attachment issue?  Or event what/where, specifically, to log?