Systemd boot options keep defaulting to LTS kernel

Ummm, yes it does. This is direct from Kernel.org (Linus Torvalds words).

Reference: https://www.kernel.org/category/faq.html

Why is an LTS kernel marked as “stable” on the front page?

Long-term support (“LTS”) kernels announced on the Releases page will be marked as “stable” on the front page if there are no other current stable kernel releases.