Is the behavior still the same with ArcMap 10.7?
If so, I would recommend reaching out to Esri Technical Support and having them update the defect to 10.7 and go from there.
To echo what George mentioned, this bug is specific to behavior that is slower at 10.5.1 than 10.3.1. It has not been address as of yet but it also would not pertain to any issues that you are seeing at 10.7. If you still see this behavior at 10.7, we need a case with Tech Support so they can replicate it here and update this bug if needed with new information.
We haven't upgraded to 10.7 yet. We wait for the 10.x.1 releases before upgrading. I was just trying to figure out ahead of time if we would experience this bug at 10.7.1.
Our users who edit annotation are still using 10.3.1 due to the bug.