I made some tests with "soak" (AI controlled) games.
Most of my custom races with ship styles (FactionShipStyleSetDefs_RACE.xml and ShipClassDefs_RACE.xml) are not researching small hulls (at least) on normal. I made some tests and if I change the the difficulty to "godlike" they are researching and have medium ships at turn 30-40, This is not a problem about missing ships or such things. The custom "core" ships are all buildable. If I take control and research small hulls myself, everything is fine. And on higher difficulty settings the AI is building these ships and crushing the other AI.
If I "soak" on normal, they do not research small hulls ever (at least not until turn 200, where I stopped the test, bored). I made tests with 7 games. They do not research small hulls.
If I play myself with a custom race and research orbital manifacturing (small hulls) right at the start and then "soak" the game everything is fine.
The custom races are mostly malevolent, aggressiv races. I do not understand, why they do not research small hulls. At turn 200 with tiny ships...
And they don't plan to research this tech, they spam (even at war) constructors and transport vessels.Not building any war ship. They spam some tiny ships at the start and then refuse to build any fighters.
This is a new problem, a few montrhs ago the same races with the same shipstyles worked perfectly.