User needs to login twice
It sounds like the FileVault encrypted drive is allowing the old password (first login) to unlock the drive. But, instead of automatically logging in as the same user, the password reset is causing the second login.

You will need to login to an local admin user and remove the user's ability to decrypt the drive on the command line...

sudo fdesetup remove -user username


Then add the user back in using...

sudo fdesetup add -usertoadd username


Or, with macOS High Sierra, you can use the "Enable Users" button located in System Preferences -> Security -> FileVault.

I do not know if this is normal after a password reset, but you may want to confirm that your password reset procedure is correct.