There might be some automation you accidentally places that's causing this, if so then try copying the loop over to another pattern and try it there.
Or, more likely the last note in the drumloop is a tiny tiny bit too long, press backspace to change the step edit thingy to none and decrease the length of the note a little to fix it. Then press backspace again to get the step edit thingy to it's default.
If nothing works then if you want, i can look at the project and see what the problem is for you. If you don't want to post it here you can contact me at
[email protected]