The only non-standard thing I can think of is my disk layout:
/boot has been mounted on a fresh 512M GPT partition.
/boot/efi has been mounted on an existing 200M EFI partition (shared with my current F29).
/ has been mounted on a fresh LVM logical volume (fedora/silverblue-root) of 18G, alongside my F29 setup. The volume group lives in an encrypted LUKS2 partition.
It seems there is a problem on EFI systems where another OS has already been installed.
If you can afford to, nuke all the existing partitions from your disk, then try recreating /boot and /boot/efi at install time to see if that helps. YMMV.