So here's *exactly* the scenario...
If I'm on iOS on my phone, or on CarPlay, OR on the web client, I'll be listening to my radio mix, and a song will come on and give me the option to add it to my library. On CarPlay there's a "+" button, on the web or on my phone I can add it to my library manually.
I will either hit the "+" (this is 90% of my usage as I listen primarily while driving), or on my phone and it will remove it from the option list (ie, the "+" disappears), and then after a few seconds -- comes right back. The song doesn't add to my library no matter how many times I click the + icon.
That's the problem. I cannot add some songs to my library at all while listening to my radio mix. The platform I'm on doesn't seem to matter at all, and I'm presuming because the backend API is responding the same way and the GUI which it's presented with is processing it the same way.