The TAdvAppStyler style I have in TDataModule does not apply to TAdvSplitter. I solve it like this:
AComp.UIStyle := dtmdBasic.appstlrBasic.Style;
It should be without it. As with other components.
Is there a TAdvFormStyler connected to the TAdvAppStyler on the form where TAdvSplitter?
This is the required setup and with this setup, we cannot see an issue here.
Again incomplete information. This only applies to subformulars
"subformulars"?
What is this? Frames? Something else?
When I couch to bed, I realized I had a typo.
SubForm-s
Please isolate this and provide a sample source project + steps to reproduce.
I did not forget 
The sample is functional. You can see what I'm describing.
I'm waiting for a response.
Your subform doesn't contain a TAdvFormStyler, hence it isn't styled.
Corrected app attached.
TMSsupport.zip (55.0 KB)
Oh, thank you for the explanation. That clarifies other things as well.