[Evolution] Searching my emails causes them all to download again

[huw <huw synapticsilence net>]

Hi

I can search emails by subject or sender just fine, but if I try to
search by message body, Evolution apparently starts downloading all my
emails again!  The message list doesn't change but a task appears at the
bottom of the screen "Retrieving message '123456' in INBOX" (the number
keeps changing).  Sure enough, a quick glance at system monitor shows
data being downloaded by the MB.

This is an IMAP+ account.  What's going on here?  Thanks in advance for
all advice.

Kind regards,
Huw