First of all, i hesitate to file this under bugs, more like FARL limitation.
i use LCCCC train, set the signal distance to 17 (or 35 tiles)
Problem was, when laying track diagonally, FARL use shorter distance, only fit for 4 train/wagon (LCCC).

So for now, i workaround the issue by using "Rail Tools" mod as a way to laying signal diagonally:
https://mods.factorio.com/mod/RailTools
It can properly placing signal exactly at 35 tiles, not matter the track orientation.

So i hope FARL can also properly placing correct signal distance diagonally.

Thank you Choumiko for this wonderful mod.

It should be improved/fixed in the latest update. Still won't be as accurate as RailTools though

Thanks, just got the in-game mod update notification.
It's okay if it's not as accurate as RailTools, as long it wasn't way too short like before.