ZW3D 2020 introduces Flexible Sub Assembly.
So if an assembly has some Degrees of Freedom (DOF) is can move. e.g. a shock absorber is models so the shaft can slide and rotate.
When the shock absorber is inserted into an assembly - by default it is rigid.
To make is Flexible within the parent assembly, RMB on the assembly in the Assembly Manager and choose Make Flexible
So here is the important part:
If the sub-assembly was created in any version prior to V2020 you may find the result is unpredictable
To fix this you need to recreate a new version of the Sub-Assembly within V2020.
Before you do that, rename the old Sub-Assembly_OLD
Create the new Assembly in the same original file as the existing(old) Sub-Assembly and use the original name(no _OLD). Remember to save occasionally.
Once completed - return to the parent Assembly and use the Change Component Option to swap the OLD for the new replacement.
You will have to Redefine the constraints to locate the Sub-Assembly.
Now you can Make Flexible the fresh Sub-Assembly and it should behave correctly.
Cheers - Paul