Could I have an undetected deadlock?

Upon running sp_whoisactive on a server for the first time, I was greeted by something unexpected:

Two sessions had been running for 13 days but both of them seemed to be blocking each other. Taking a look at sys.dm_tran_locks:

The configuration value for the blocked process threshold setting is 10 seconds. Other deadlocks are being successfully resolved on the server via the deadlock monitor.

Information from the @get_locks parameter:

I've occasionally seen the term "undetected deadlock" but I don't have any direct experience with them. My questions are:

• Could this be an example of an undetected deadlock? I can't see how the situation gets resolved without intervention from the deadlock monitor thread, but for some reason that hasn't happened yet.

• Is there anything to do other than update to the latest CU and hope the issue doesn't occur again? The server is currently on 2017 CU10 which I know is a bit out of date.

From a comment left by David Browne - Microsoft

"Undetected deadlocks" usually means "undetectable deadlocks", eg where session is blocked by a SQL Server lock, and the other is blocked in the client application, or where a session calls xp_cmdshell which a runs a program that connects to the database and becomes blocked by the session waiting on the completion of xp_cmdshell. Having an ordinary deadlock not resolved automatically looks like a bug.

StackExchange Database Administrators Q#299736, answer score: 2