The following script works: works.txt (attached)
And the following script doesn't: doesnt.txt (attached)

The only difference is the "-lts" in line 4. Specifically, what "doesn't work" is that it's looping on "Automatic boot in 5 seconds" in the syslinux menu.
I'm not 100% sure that it's related to syslinux, but it seemed to work if I switched to GRUB.
I spent sooo much time trying to pin down the cause of the issue, especially since it was my first time using Parabola, so please, I think we need to add a comment on the official installation guide ASAP linking to this issue.

PS: The issue is still present if I use UUIDs in my syslinux.cfg, or if I use a separate boot partition, or if I install syslinux manually. I did not test with GPT instead of MBR.