Hopefully by the time you’re reading this, there will have been an update that fixes this (I sent them info on the bug and got a response back.) But just in case…
Problem: pressing Command-V on the keyboard doesn’t paste any data from the clipboard into the Transmit Disk menu bar.
Workaround: Right-clicking (or Control-clicking) in the password field presents a small context menu, and choosing Paste will actually paste the contents of your clipboard.
The workaround was actually provided back bu the Transmit team.
Since I use Lastpass and use copy and paste for really complication passwords, being able to paste is vital so I thought I would share for anyone else who has run into this.