iPhone Podcasts consumes very high storage

Podcasts consumes abnormally high "Documents & Data" storage. I deleted Podcasts app and reinstalled. Then "Documents & Data" usage dropped almost Zero. But it became over 1.7GB now. I deleted cached data after I listened but storage use does not drop as much as the cached podcast I deleted. As result, storage usage steadily increases. As you see following screenshot, I have only 10.4MB cached podcast, but "Documents & Data" shows 1.72 GB usage.

I use iPhone OS language in English. But most of podcasts I listened were Japanese. So, I wonder Podcast app cannot delete cached podcasts properly if their files were in double byte characters and OS language is single byte language like English. Does anyone have same issue? Is this known issue?