room stays unread if no messages due to retention policy

[matrixbot]

This issue has been migrated from #10787.

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Initial report here: element-hq/element-web#14976

Description

If older messages gets removed due to retention policies and no new messages after retention time, room becomes unread forever until new message is inserted.

Tested clients: Element (web, Android and iOS)

Steps to reproduce

• make new room
• set retention policy to short time
• send message and wait until retention kicks in
• clear client's cache and reload
• room stays unread

Version information

• Homeserver: kvlt.ee

• Version: {"server_version":"1.41.1","python_version":"3.7.3"}

• Install method: pip

• Platform: Debian Buster