iOS 11 - The Home button won't bring up passcode when the phone is in a wet weather cover

I have a wet weather cover for my phone - I can see everything and before iOS11 I was able to bring up the passcode entry (as TouchID won't recognise my fingerprint).

Since upgrading to iOS11 (both 11.03 and 11.1) this doesn't work. I press the home button and nothing happens. I hold down on it and Siri starts.

If I take the phone out of the bag then TouchID works.

However, *sometimes* if I have the phone out of the bag and try to press the home button with a finger that is *not* registered in TouchID then the same behaviour happens - the passcode entry doesn't come up.

Seems like a bug - has anyone else seen this?