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

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?