In the Samba module there is an option to “Convert Users” - ostensibly to convert existing Linux users to Samba users. I struggled with this then when I went to my user info page in Webmin and manually changed the password, the access to the Samba share finally worked.
It appears that Webmin is not grabbing the correct password info from Debian and requires manual intervention.
If Samba has its own passwords, Webmin can’t just copy existing passwords over. Webmin doesn’t know the password and the password cannot (reasonably) be recovered from the hashed passwords in /etc/shadow. If Samba can use PAM for authentication (I don’t know, it’s been 20 years since I’ve seen a Samba server), then nothing needs to be copied.