This issue was first reported in ISE version 3.1 patch 1, in p1 even local admin couldn't import the csv file. After p2 upgrade, this issue was gone for local admin, but issue still existed for SAML administrators. When I raised a TAC, Cisco is claiming that the issue will be resolved in P6 which is due in March 2023. Don't understand why it takes 6 patches to update a single bug and couldn't be resolved in next immediate release as soon as the issue was reported.
