Hello again, @Allan!
I'd still wouldn't recommend doing this with the SSHD, especially if there's important data on the drive. I'm afraid that the procedure you have executed might have damaged the SSHD beyond repair. Mechanical hard drives are very sensitive and there are tiny holes on the cover but they are not large enough to allow any dust to be accumulated inside the cover, so it was unnecessary to open it. For hard drive physical operations, it's essential to use a specific room (dust-free environment) otherwise the surface of the platter or the head would most probably accumulate dust and get corrupted.
I'd advise you to consider replacing this SSHD.
Best of luck!
SuperSoph_WD