Apple Podcats app won't play all of downloaded podcast when not connected to wifi or mobile data

iPhone 7 running iOS 15.8.2.

This morning, I downloaded a podcast to my iPhone through the podcasts app over wifi. It's showing the arrow symbol to show it's downloaded, it's appearing in my downloaded episodes list and in my detailed storage information. I listened to it on and off through the morning, while connected to wifi, then carried on while walking my dog, not connected to wifi and with mobile data turned off. The podcast played fine up to a certain point and then stopped, showing only a buffering symbol. Skipping back 15 seconds, it played again but stopped at the same point. It would not start again until I'd reconnected to wifi or mobile data.

Looking more closely at the timeline, it's clearly only loaded in a certain amount of the podcast, with the last fifth of the timeline greyed out. Which makes no sense for a podcast that is, by all accounts, stored locally on my device.

It's a free pod, it doesn't need any kind of subscription verification. Its size is stated to be 300mb (which is the same size as when downloaded through iTunes to my desktop) and I have about 4gb of free storage on my device. There are no settings in the apps' settings that suggest offline play is disabled or that downloading and/or storage are restricted.

Why is this happening? Why would the app need internet access to play all of a file stored locally?