[exim] exim behaving strangely when writing to maildirs? Or …

I have had to reorganise my systems due to a hardware failure, and now
my home directory and Maildir reside on the same machine. I had not
noticed the problem I am now experiencing prior to this.

I am getting all mail in my various folders, except my main inbox.
Whenever I try to retrieve mail in that, it fails, with dovecot logging
an error like this on the newest file:

Jul 26 14:56:08 fitzalan dovecot:
imap(phil)<26727><ZKUF05WOtpcgAQiwDOMAASJonf/+Bp+v>: Error: Mailbox
INBOX:
open(/home/phil/Maildir/cur/1564133392.H197929P18087.fitzalan.tinsleyviaduct.com:2,)
failed: Permission denied (euid=1000(phil) egid=1000(phil) missing +r
perm: /home/phil/Maildir/cur/1564133392.H197929P18087.fitzalan.tinsleyviaduct.com:2,)

At the time, it seems the relevant file is owned by root, yet ownership
is changed to phil before long. This looks rather like a problem with
the way mail is delivered to the inbox by exim. It could, instead, be a
misbehaviour of dovecot, but in any case its main effect is that I have
an empty main inbox in my MUA.

Is this an effect of any behaviour of exim? If so, is there a way to
configure it not to do it that way? I can quite happily get details
from exim's logs if required.

My setup is currently running Debian buster, upgraded very recently
from stretch.

Thanks in advance for any advice on this.