I bought a Magnavox MDR515 myself, to study what the issue is.
Have been working on this for many hours now. It involves, not only the CGMS line 20 capability but, also using line 21 field 2 XDS capability.
That is, the CGMS data can also be included in line 21 fiels 2 using Extended Data Services protocol. This is all part of CEA-608B Standard. After checking out that I am complying with that, the Magnavox is still intermittent when recording.
If I remove the colorburst component of the S-Video signal, for example, the unit records fine, which tells me that the unit is also looking at colorstripe although I have not actually seen it in the video source.
So, I am still working on it.