Change in behaviour of Time Machine Back Up

Machine iMac late 2013 OS Catalina 10.15.7

The behaviour of Time Machine has changed. I back up to 2 USB connected external hard drives. They have always been encrypted & password protected. The drives would be recognised by Finder after start-up or if added during a session. Most times the Time Machine process would resume. Very occasionally I would get a dialogue box asking me to unlock the drive by entering the encryption password. I didn't. I clicked cancel and Time Machine back-ups went on as normal. The drives would update to the correct last date and time of back-up in the preferences view of Time Machine.

Now when a drive is connected the dialogue box always appears and the password must be entered for Finder to show the drive as connected and for Face Time to back-up. A drive attached at the time of power on must be unplugged and then replugged into the Mac.

Having finished back-up the date and time in preferences do not change. The back-up on the drive is up to date.

So, has the recent Catalina update corrected a faulty process eg no need to enter the password or has the Time Machine process gone a bit flaky?