We're sorry for the troubles! There indeed seems to be an issue with Spine 3.8.99 exporting (at least your set of) IK curve types incorrectly in binary data export. This issue is already no longer present in the earliest versions of Spine 4.0.
Since Spine 3.8 does not receive any updates any longer, I'm afraid I can't offer any bugfix for this export-related issue, other than either using .json export if you don't want to work around this limitation, or to upgrade your project to Spine 4.0 (and re-exporting your assets from Spine 4.0). Sorry!
