We've had this issue for quite awhile, and it has spanned from v5.6 to v6.1. It doesn't matter if the Host machine is running Windows 10 or Windows 7. The issue is if the Windows Host machine is locked and unlocked while connected to a guest machine, then when you try to type on the guest machine it will act as though you are holding down the Windows key. The only way to "fix" the problem is to tap the Host's Windows key to release it.
I just found out today that this also happens if the guest machine is a Mac. If you go through the same procedure of locking and unlocking the Windows Host machine, then the Mac guest machine will act as though the Command key is stuck.
Customer support service by UserEcho