It seems after opening a .glyphs file that was saved by Glyphs 2.3, re-interpolating brace layers is ignoring bracket layers. It does not matter which version of Glyphs I use to re-open the file.
However, if I freshly open a file saved by Glyphs 2.2 in Glyphs 2.3, re-interpolating still works. Strangely, even re-saving the file with Glyphs 2.2 seems to “repair” it and re-interpolating works again in all versions (if freshly opened). So, this seems to be related to changes in the file format (the layer names are omitted, right?) but the re-saving trick suggests that this info could be restored on reading the file.
Oh, and re-interpolating brace layers in Glyphs with caps crashes Glyphs (879) in all cases.