not only U mobile,i'm using Celcom also meeting the same issue. Because 4G is using 2.6Ghz which has smaller coverage per cell tower and no any mobile service providers have yet covered seamlessly. When you choose 4G first,your phone will prioritize to search for 4G signal first and if that fails,then search for 3G and 2G. That process takes time,especially when moving. I think it's normal in current stage.

it could happen in some area when 4G is weak and so most phone will drop the signal and search for alternatives. That will cause some delays to establish the connections to cell towers and all those phones are contacting the cell tower concurrently and there's some limit at one time.