As others have stated, some properties, methods, and events (it seems like), cannot be written to, or read from, when desynchronize (aka running in parallel)
In the documentation, you want to look for these tags to see if something can be read or written to when desynchronized
Another thing to not is that your use of parallel is not good. Currently, you are only running 1 parallel thread, which means you cannot have multiple parallel threads running at the same time. You can have more parallel threads by having multiple actors running scripts in parallel
Parallel lua can also end up being slower than serial for work that isn’t significant. The overhead of transferring parameters and running the threads in parallel can be bigger than the benefits of running multiple threads at once
While the structure of actors has been made for having separated instances (like npcs), with their script, run in parallel, as of now, very little properties are write safe (as of now), which makes this use case umm not great.
As for running mathematically expensive stuff (aka doing a lot of raycasts, spacial queries, or compression/decompression of really big strings/tables), I have made a module that makes it easier to use, as the actor system isn’t adapted for this use case