Question about LTE OnOff app

[legaldeejay]

I have LTE OnOff app which allows you to switch between 3G and 4G. The app recommends to set it to "CDMA + LTE/EvDo auto" to allow phone to switch between 3G and 4G by itself. But sometimes it goes to 3G and won't go back to 4G. So when I go into the app, the preferred network apparently switched itself to "WCDMA preferred" or something else. When I set it back to "CDMA + LTE/EvDo auto" it goes back to 4G. Any explanation for this?