I created a long playlist of music on the web interface, then "play shuffled" it in dsub. After listening to a bunch of those songs, I added several podcast episodes via "Play Next" . They added correctly, where I'd expect, and played when expected (although as a separate issue, sometimes if you play next when a song is almost over, it skips over the next track. This has happened for a long time).
After listening to them, some but not all of them disappeared from the playlist. "Remove played songs" is not checked.
Also, I accidentally chose play last for one of the podcasts. I dragged it way up through several screens of songs. That appeared to work, but when I let go and scrolled back down to the bottom, the podcast is still there. I then scrolled back up to where I moved the podcast, and it is gone. Just now, to make sure this was still happening, I did it. It happened, but then after the screen timed out, the track moved to where I moved it to.