There is a report that the mouse pointers disappears sometimes. Someone upstream reported that it was seen with one user profile but no with another on the same server. Some hints upsteam that it might be related to that the user uses text scaling (HDPI).
Lots of mouse pointer issues has been fixed upstream and brought to ThinLinc via vendor drop r32951. This too, should be fixed.
Even though I never was able to reproduce the original problem, I feel quite confident that it is fixed. Have tested many different scenarios including HDPI scaling and mouse shadows. The code looks good as well.