I might be wrong that it's the Syncthing install that changes the password.
As part of the session, before using the Syncthing install option, I used the Samba install option which did ask me for my samba password. In my case, my samba password is not the same as my linux password.
Here's part of the script for the samba install. It looks like the middle line is changing my linux password to my samba password which would cause the problem I saw.
```
useradd $SMBUSER
echo -ne "$SMBPASS\n$SMBPASS\n" | passwd $SMBUSER >/dev/null 2>&1
echo -ne "$SMBPASS\n$SMBPASS\n" | smbpasswd -a -s $SMBUSER >/dev/null 2>&1
```