Bugzilla

No, the ones you've noted are not causing any issue. The behavior there would be pretty much expected with the current implementation.
What jumped to eye, was the fact that after the whole block/unblock scenario; the page refresh did not appear to reset the status of the 1st request keeping it as is.

However, really good point there.
In an attempt to reproduce it with your STR, I've noticed that adding a refresh between the block/uncheck causes the issue to manifest again. 
The summary might need to be adjusted as well or it might be another issue .

So:
1. Launch Firefox, enable DevTools - Netmonitor - Blocking tab;
2. Access https://twitter.com/
3. Right Click and block the GET method for the html document (1st in the list);
4. Refresh the page;
5. Uncheck the checkbox of the create entry in the Blocking panel
6. Right Click and block the GET method for the html document again. Nothing happens in the Blocking side panel -> BUG

No, the ones you've noted are not causing any issue. The behavior there would be pretty much expected with the current implementation.
What jumped to eye, was the fact that after the whole block/unblock scenario; the page refresh did not appear to reset the status of the 1st request keeping it as is.

However, really good point there.
In an attempt to reproduce it with your STR, I've noticed that adding a refresh between the block/uncheck causes the issue to manifest again. 
The summary might need to be adjusted as well or it might be another issue, this part might need confirmation from you as well.

So, the STR for your scenario would be:
1. Launch Firefox, enable DevTools - Netmonitor - Blocking tab;
2. Access https://twitter.com/
3. Right Click and block the GET method for the html document (1st in the list);
4. Refresh the page;
5. Uncheck the checkbox of the create entry in the Blocking panel
6. Right Click and block the GET method for the html document again. Nothing happens in the Blocking side panel -> BUG