I appreciate you bringing this up and sharing the illustrative plots. This has come up before, and it’s tricky because I can’t think of anything in MoorDyn that would cause a change in behavior partway through a simulation. Other instances of this are documented in these threads:
When the change occurs seems to depend on the time step size, and it only seems to be a problem in some versions of FAST/OpenFAST.
Could you let us know what version of FAST/OpenFAST you’re using, and what MoorDyn time step size (dtM) you’ve set?
If this is occurring with the current version, we will try to replicate the problem and investigate further.