I came across this particular issue in Mendix Modeler 5.21.1 and I was unable to solve it in any reasonable way.
It is a problem in the Modeler, but yes it is affected by the performance of the environment (more probable to appear in low-performance environments), that's why reproducing it for the Mendix support team was a pain; however, I was successful at doing that by running heavy processes (duplicating huge files for example).
The answer I received by them was that they were able to verify it using the sample application that I provided, and that it was fixed in Mendix Modeler 7.6.
Seeing that you are using Mendix Modeler 7.7.1 and are still facing the same issue makes me question their reply.