All local folders lost in KMail

Hello list,

Many times I've had to create a new user for myself and set KDE up again from 
scratch, always because KMail has got itself into some bind or other. I did it 
again yesterday because KMail filtering was generating a duplicate of every 
message it filtered, and even when I deleted the duplicate it would often pop 
back into existence.

Having set everything up again, with a single IMAP account at a local server 
set to download all messages, I started manually applying filters to each 
unread message in the in-box. Each filter is set to mark the message unread 
while moving it, but on yesterday's system it was doing that and then 
immediately marking it read. Odd, but not life-threatening.

This morning I came back to the machine, which runs 24/7, and started KMail to 
find that every one of my local folders had vanished, leaving 11 messages in 
the IMAP in-box.

So I had to import yesterday's archive and set up the properties of 30-odd 
folders once more.

I swear I did nothing to meddle with any databases or my home directory. How 
can this possibly happen?

-- 
Regards,
Peter.

Gentoo stable system, openrc-0.41.2
gcc 8.3.0, sys-kernel/gentoo-sources 4.19.57
QT 5.12.3, KDE frameworks 5.60.0, KDE plasma 5.15.5
KDE apps 19.04.3 incl KMail 19.04.3 (5.11.3), akonadi 19.04.3
dev-db/mariadb-10.2.22-r1, net-libs/webkit-gtk-2.24.2
x11-drivers/xf86-video-amdgpu 19.0.1
dev-libs/amdgpu-pro-opencl 19.10.785425-r1