This problem, podcasts stopping at 20 seconds typically, also seems to affect the HEOS app, i.e. the Denon/Marantz' Home Entertainment Operating System on my iphone SE. The HEOS phone app is the control panel for podcasts streaming to my Marantz MCR612 receiver. The problem case is not streaming from local storage on the phone, i.e. from a fully downloaded podcast file, but rather streaming to the Marantz, which has HEOS internally as well. I can easy stream all stations, meaning streaming with no start and stop times displayed, but I cannot stream podcasts, streaming with a start and stop time, at all generally. This is all obviously very complicated, but the key clue and commonality between our two cases is stopping after 20 seconds typically, not always. Podcast failure at 20 seconds in both your case and mine is too big a coincidence to not be related to a causal factor. So something in the iphone must be disrupting podcasts (not stations) whether on the native Apple podcast app or on Denon's HEOS app, and likely on any podcast app on the phone. Upgrading to IOS 15.0.1 did not help at all, so if this is something Apple can/should fix, they have not got to is over the summer to early fall months. Like you, I can play podcasts without problems on Apple's Podcast app, which is not networked, so wifi and cellular are not essential.
