APIM HEAD methods no longer working across multiple APIM instances

Question

We have an APIM instance setup which is proxying 2 calls to a file storage account. The GET method is working as expected The HEAD method is no longer working

We are also seeing this across other APIM instances that are calling a HEAD endpoint pointing to a function app. It is the same behaviour hangs and eventually fails.

There have been no changes to this except a cert which was installed last week and it working as expected.

This stopped working around 19th/20th of November as we have successful calls logged on the 19th.

This behaviour is inconsistent as it is working in our CI & Prod environments but not our Test and Pre-Live environments.

If we call the File Storage HEAD endpoint directly it works. The issue seems to be APIM -> File Storage

I have looked in the exception logs and during the call I can see lots of these errors: [{"id":"0","outerId":"0","type":"OperationNotFound","message":"Unable to match incoming request to an operation.","parsedStack":[{"level":0,"method":"configuration","line":0}],"severityLevel":"Error"}]

Then after exactly 5 minutes I can see a different error: Client connection was unexpectantly closed.

Looking at the traffic in the File Storage account I can see a 200 was returned for backend call from APIM -> File Storage so traffic is able to get from APIM to the file storage.

We also have this issue with a similar HEAD endpoint in another subscription which is different solution to this one. We have multiple instances of this endpoint in other environments and these are all working correctly.

The 200 returned from the File Store but times out in APIM after 5 minutes