IAS prompting some users for login details

Hello,

We setup IAS for a client and they have been using it for a bit. They recently raised an issue where some of their users are getting prompted to login at the start of their session but some users do not. Has anyone else come across this issue?

They have 2FA setup in their Azure system. They all are employees in IAS but we can't seem to see any differences as to why some get prompted and some don't