At least for Ubuntu, this was a change in policy from a couple of years ago. This Discourse post from August 2024 mentions it:

To provide users with the absolute latest in features and hardware support, Ubuntu will now ship the absolute latest available version of the upstream Linux kernel at the specified Ubuntu release freeze date, even if upstream is still in Release Candidate (RC) status.

And Ubuntu 25.10 did it last year:

Due to the final upstream release occurring after Kernel Freeze, the kernels shipped with the release images will be based on 6.17-rc7. Updates for all Questing Quokka kernels are scheduled for release in the subsequent week to incorporate the final upstream 6.17 release.

score 1 · canonical