When server leaves room check for stale device lists. (#6801)

When a server leaves a room it may stop sharing a room with remote
users, and thus not get any updates to their device lists. So we need to
check for this case and delete those device lists from the cache.

We don't need to do this if we stop sharing a room because the remote
user leaves the room, because we track that case via looking at
membership changes.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/6801.bugfix b/changelog.d/6801.bugfix
new file mode 100644
index 0000000000..f401fa5d69
--- /dev/null
+++ b/changelog.d/6801.bugfix
@@ -0,0 +1 @@
+Fix bug where Synapse didn't invalidate cache of remote users' devices when Synapse left a room.