Are you running on 5.0 GP version?
There are certain changes in this version of iOS if compare to older version. The certificate location of certificates in iOS 12 and newer is different than older iOS version so you need to re-import certs.
Please refer this -
Also https://live.paloaltonetworks.com/t5/General-Topics/GlobalProtect-5-0-for-iOS-12-and-User-Certificat... talks about same issue.
Hope this resolves your issue!
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