Encrypted drives are usually decrypted at boot, so if the system is running live and the drive is unplugged while "hot" and plugged into another system will the data still be unencrypted?
I may be misunderstanding the implementation here, but let's use a LUKS encrypted drive as the example. Is the decryption handled in RAM or something like that (as opposed to decrypting the underlying, smaller mapper)? If this is the case, then it wouldn't matter if the drive was taken out because it would not be decrypted, whereas if it is done on the mapper for the encryption (written as decrypted) it would likely persist as decrypted between swapping the drive.