Okay so it has been a long while with the issue with OTP Session template, and currently I tried self-hosting and found out that it is linked with Verification template in the console, any changes in any of the template updates both. But the email for OTP Session comes with default template only.
TL;DR
Issue: OTP Session template not working and is linked with Verification template, causing changes in one template to update both.
Solution: The email for OTP Session is only coming with the default template.