Master Password entry cursor disappears


I’m running BW on Firefox/Windows and after a browser restart at some point I note the exclamation mark on the BW icon so I try to sign in using the master password. But after the sign in box appears I click by the entry box and start typing the password - after I’m finished I look up (yup I’m not a great typist) to find that my password is not being entered.
Another click and the cursor appears and I can type it in. I can’t seem to replicate this reliably unfortunately - sometimes the cursor is there, sometimes not.

Has anyone else seen this? It’s rather a large security risk if my master password is being captured by another process …