Simulink support for Uint64

In R2019b (Simulink update 3, I believe), I have set signals in a Simulink model and in the data dictionary to uint64. The model only contains integer and boolean data types. (There were doubles in the model previously, but these were all changed to uint64 and checked to see that none were missed). After diagram update, Simulink is automatically changing the uint64 signals to ufix64. Not sure why that is happening - Is the fixed point tool doing that ?? Shouldn't I see uint64 for these signals on the data type size overlay ??

In the test sequence in the test harness, values are being assigned to those signals whose data types are cast to uint64(x) in the assignment statement. The diagnostic viewer is flagging a type mismatch as unable to assign a uint64 to a ufix64. The offending lines are fields in a structure, which is inside another structure. Another uint64 assignment line, which is just a field in one structure, does NOT give an error. Can this error be related to nested structures ??

Also, the uint64 type cast on the offending line appears to be in bold blue, whereas the type cast for the other data types are not in bold. Is bold on a data type in a test sequence line indicative of something ?? Could not locate any documentation info for that specifically.

Any assistance that you can provide is appreciated. Thanks.