For some reason contours of \u11BC when used as an accent in some glyphs in
Korean fonts from gulim.ttc cannot be always algorithmically emboldened.
See a Swing reproducer.

DotumChe is an example font from gulim.ttc collection.
Here for instance \uC591 contains \u11BC as two contours (out of five) in glyf table.
DotumChe does use embedded bitmaps. In its EBDT table \uC591 has a single component.
Here oval depicting \u11BC part is never bold.
The same effect is visible in \uD589 etc., defined the same way.

Unlike that, \uD535 has in EBDT table references to other glyphs, \u11BC too,
but no contours defined in glyf table.
For \uD535 oval of \u11BC is bold when required.

An immediate change leading to this visible difference in boldness is JDK-8214002,
together with JDK-8204929.