When I open a document with a Layer Group set to Knock Out (Shallow in this case), it appears to not be working (i.e., not knocking anything out), but when I simply click on the Layer Group to open Layer Styles, the knock out suddenly appears correctly. However if I then save the doc and reopen it the problem is back.
Also, I frequently make Knock Out Layer Groups which contains other Knock Outs within them (i.e., punch a hole in a full-canvas Knock Out layer). These do not seem to be working either (at least not in legacy documents), although I've had limited success in rebuilding them from scratch.
This is on Mac OS 10.14.6 (Mojave).