Most likely it is the same backend schema mismatch reported here: Approval Error after access request approved.
Your GET response includes two newer fields, machineIdentityManagerAssignment and circumventApprovalProcess, that were added to the ApprovalConfig document by the March 2026 “Delete Accounts (Human and Machine) with Configurable Approvals” release.
In the related thread the approval service throws UnrecognizedPropertyException on machineIdentityManagerAssignment, and the 14 known ApprovalConfig fields listed in the error do not include it. That thread errors on the post-approval path and yours errors on submit, but both point to a SailPoint consumer service not being updated for the newer approval config shape. Not a bad entitlement or access profile. You already tested three different request items and the common factor is this config.
Open a SailPoint Support case and include the failed submit response from browser dev tools if you can capture it. That confirms whether the submit path is hitting the same UnrecognizedPropertyException.
Hope, this helps 