Can you debug this some more and try to provide us with a little more data on what's going on?
For example, if you execute your usual command with
snap run --shell your.command you'll find yourself in a shell inside the snap environment, with the same permissions of the actual command you were about to run. From there, if you do
ls -l /dev/tty you should find the device available, and you can also run the actual
sshpass command as well to explore how it behaves.
Please let us know how it goes.