Read Only Today but Worked Yesterday

I bought a new WD My Passport Ultra 2 TB to be used for Time Machine backups on my MacBook Pro and my wife’s MacBook Air. I reformatted the drive to be used for a Mac by following the instructions on the WD web site. Yesterday I was successfully able to backup both of our computers onto this external drive. However, today when I open the drive, I see that both of our backups are “read only”. In Disk Utility it shows APFS Volume (Case-sensitive, Encrypted), it does not show NTFS. I did change it to Mac OS Extended Journaled like the instructions said, but today it does show it as APFS. How did this happen overnight and how can I fix this? Thanks-