First, please go to "Settings" -> "Mobile Data Usage", and make sure all the options (Stream, Download and Show Episode Covers) are turned off. You can also choose to go to your phone’s mobile data setting, and disable Castbox to use your mobile data.
If it doesn't work, we need more info to further locate this data-consuming issue and will forward the info to developers for evaluation.
1) Please show us how much data is consumed during a certain time span (day/week/month).
2) When this problem happens again, please go to your Castbox "Personal" tab - "About" - "Contact us" - "Submit report via email..." and send us an email in this way. Then we could get to know your log for further analysis.
3) Did you usually tap the playback button on the preview half page to play episodes? Or you tap the episode cover/player bar to play them?
4) For the downloaded podcast you listen to, did you download them manually? Or are they auto downloaded?
5) Did you turn on any auto delete settings? There are two auto-delete settings for the app. One is "Automatically delete from Playlist" and the other one is "Automatically delete played episodes". You could find both of them by going to the app Settings.
6) When you opened the app or played episodes, did you get a pop-up notification asking your permission to use data?
7) Please check if you could play the downloaded podcast normally when you are offline (without mobile data and wifi).
We are working on making our app better and our support team will be glad to help you out.
Thanks for your patience and cooperation. : )