Hi,
did you have any luck with changing the behavior of Okta? For 15.1.x it seems that the issue is still not resolved and while reconfiguring the Okta IdP there is only the Key Transport Algorithm, which doesn't change anything.
The issue is tracked under ID 485387 (https://techdocs.f5.com/kb/en-us/products/big-ip_apm/releasenotes/product/relnote-apm-11-5-0.html#rn_ki_apm_1150), which is not listed in the Bug Tracker.
From my understanding it seems to be not possible to encrypt the assertion between F5 BIG-IP and Okta. Or is my understanding wrong?
Regards