I found this when experimenting more with #10078. I guess it is related but not the same.

When a package contains only classes, @group doesn't work.

/** @group foo */
class Bar()

/** @group bar */
class Baz()

class Other()

!only-classes.PNG|thumbnail!

If I add an object, the groups work:

/** @group foo */
class Bar()

/** @group bar */
class Baz()

class Other()

object NotClass

!classes-and-objects.PNG|thumbnail!

(I also noted that now the "only show superclasses on hover" effect is disabled. On purpose?)