Can't generate Big/VeryBig Oil Field, bug or known issue ?

[waduk]

0.12.24

I can't generate Big/very big oil field, regardless what setting, the result stay the same.
The setting MAY look working in starting area, it generate according to setting, but outside the starting area, the map can only generate small oil field.

Yes, i already check, i scan 100x100 chunk wide on numerous attempt, the result is the same. On average it only generate 6-7 oil patch per field.
On starting area, it can generate 15+.

Yes i know about RSO, playing on it a lot, but this is about vanilla factorio. I want to play purely on vanilla.


Btw, the copper, iron, stone and coal were doing OK-ish. It's only crude oil spawn outside starting area that have the problem.