What would cause a certificate installed on iOS to be invalid?
Several things could be problematic.


the device - you can swap the certs on the devices to test that
time settings can cause a cert to fail to be authenticated if it uses some sort of OCSP/CRL revocation check
iOS being more picky than Mac and there being something technically wrong (or just not what iOS wants even if it's technically a valid cert)
provisioning profiles on one or both device that alters the trust policies or networking in a way that blocks some intermediate cert needed to validate the problem cert.