Losing 4G connection

[legaldeejay]

I have the app LTE OnOff which allows you to set Preferred Network Types. This is the list of options:

0 WCDMA preferred
1 GSM only
2 WCDMA only
3 GSM auto (PRL)
4 CDMA auto (PRL)
5 CDMA only
6 EvDo only
7 GSM/WCDMA, CDMA, and EvDo (PRL)
8 CDMA + LTE/EvDo auto
9 GSM/WCDMA/LTE auto
10 CDMA/EvDo/GSM/WCDMA/LTE auto
11 LTE only
12 LTE,CDMA

I have it set to CDMA + LTE/EvDo auto which allows both 3G and 4G. What often happens is that I will lose 4G and the preferred network will switch itself to something else, like CDMA auto (PRL) and stays on 3G until I switch it back to what it was before.

Any explanation for why this is happening?