This is a real bug bare for me. On Vodafone and i expect the other networks if i am streaming my fav radio station and i loose 4G my handset falls back to 3G 99% of the time it never reverts back to 4G (which im playing for) unless I physically stop the player.
This really does need addressing, I cant believe it has been over looked by the networks. It seems my phone needs to be idle for 10 seconds or so the switch to take place???!!!

It seems the handover from 3G to 4G cannot take place whilst data is transferring over 3G. I notice this all the time as I'm right on the edge of 4G coverage on Three, it has no problem going from 4G to 3G and continuing to transfer data, but it will not jump back up to 4G until data has stopped being transferred.

It can be very annoying when I'm trying to watch a YouTube video, even 4G with a terrible signal can stream without buffering but as soon as it goes back to 3G the video buffers throughout and the only way to get back to 4G is to stop the video.

I assume it's some sort of technical limitation, but I don't understand how or why.

I get exactly the same on Three. I am coming to the conclusion that its just another "feature" of 4G LOL

I thought it was ios but it seems android does it also,
Your issue is voda3G is often congested.

Yeh its been like this since 4G was rolled out by all networks.

It's the same as 3G to 2G handover during a call. It'll then stay on 2G until the call is completed, regardless of having 3G available.