I know this is an old thread but I have started having the same problem as Rod except that the emails aren't going to the Junk folder.
Just recently I noticed that if I subscribe to a thread I did not get email notifications of further responses to that thread. This occurs on both Firefox and Safari and started before I updated to Catalina!
I have checked General Settings and the Default Thread Subscription Mode is ‘Instantly, using email’.
I have checked My Subscriptions and I only have one thread listed as subscribed, which is correct. The tick box for Instant is unticked and the tick box for Notifications is unticked. If I tick them both they disappear when I check back.
Below the tick boxes is a drop-down menu called Selected Threads. If open it and tick Update Subscription Type for ‘Instantly using mail’ then hit Go the tick boxes for the subscribed thread disappear.
I'm probably doing something silly but haven't been able to sort it out
Update: I have subscribed to this thread and set it for instant emails. When I check the list of subscribed threads this one appears but is not ticked for notification.