I don’t think that it should differ. I would say that IFSAPP shouldn’t do it.
Have you checked if there are any differences in the grants for users created by IFSAPP or another Admin?
Should users only be created by IFSAPP? Short answer is no.
In IFS hosted environments where you won’t have the IFSAPP credentials it is perfectly fine to create IFS users with IFSADMIN.
Assuming we are talking about logging into IFS EE, the created user should have the FND_ENDUSER permission set granted as a minimum, which in turn contains FND_RUNTIME, this permission set has the CONNECT privilege and you will need this to connect to IFS Applications.
No. There is no need to do it as IFSAPP at all and I would recommend not doing that - use separate accounts as it appears that you are.
Nick