Separators are almost invisible in 200% as they don't differ by much from the normal padding around the toolbar buttons. Unfortunately it's not clear what can be done about this -- we don't change the size of the normal separators, so it looks like a bug in the native control itself (seen at least under Windows 10.0.19042.867).

The only solution I see is to handle all separators as stretchable ones, i.e. manage them entirely ourselves, but this doesn't seem very appealing.