When using Garmin Express on Catalina it is not possible to manage the podcasts that are on my Vivoactive4. The podcasts that are on the device are listed using their MACOS filename (a bunch of random characters), which means it is not possible to identify which ones to select for removal.
Podcasts on the computer are correctly displayed using the podcast author/episode title - I assume this info is pulled from the Podcast app on mac. But once the files are on the watch they are garbled.