UpToDate and out-of-sync inconsistence

Hi all,

On drbd-9.0.23 (CentOS 8.2), we are facing an inconsistence of node status.
That is, although the state of both DRBD device is UpToDate, there exists
some out-of-sync value.

#drbdsetup status --verbose --statistics
 r008 node-id:0 role:Primary suspended:no
 write-ordering:flush
 volume:0 minor:8 disk:UpToDate quorum:yes
 size:109757124 read:3065 written:105640 al-writes:27 bm-writes:0
 upper-pending:0 lower-pending:0 al-suspended:no blocked:no
 fs100 node-id:1 connection:Connected role:Secondary congested:no
 ap-in-flight:0 rs-in-flight:0
 volume:0 replication:Established peer-disk:UpToDate
 resync-suspended:no
 received:0 sent:105028 out-of-sync:12 pending:0 unacked:0

I see this issue on several systems running drbd-9.0, and I made a stable
reproducer (attached for reference). This repeats umount/mount of DRBD disk
on primary node, and disconnect/connect on secondary node at the same time.

When I run this reproducer on drbd-8.9 for comparison, I never see the
issue.
On the other hand, I see this not only drbd-9.0.23, but also the latest 9.1.

Am I doing something wrong or do we have a bug in out-of-sync calculation
and state transition to UpToDate?
Any feedback would be greatly appreciated.

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