I understand you have experienced issues with the Post Login action not triggering for new users.
Before we continue, could you please clarify how you are assigning default roles to a new user? Would you be able to share your Action script for clarity? (Be sure to obfuscate any sensitive information).
Next, I have not been able to reproduce your observations but instead can confirm that the Post Login Action scripts execute after the Post User-Registration Action scripts. Seeing that, I recommend adding a console.log() statement to your Action scripts and using the Real-time Webtask Logs Extension to verify if your scripts are executing.