I think I encounter the same issue as the original poster but on CHR 7.19.1 will file a bug report
I think they said it is fixed in 7.20.
I see that’s cool if that was the case 
I had an issue with RoMON on devices which were behind a switch after v7.17+ and now after updating to v7.19.2 the issue is gone for me.
I already update the CHR to 7.19.2 but still no dice the router is behind switch running on 7.19.1 the CHR is working as it should it’s just that it can’t be see as part of romon neighbor