I have the same issue, I don't get why WCDMA preferred exists and not an LTE preferred (in *#*#4636#*#*). I'm running stock everything, only thing I've done is rooted, and I'm updated to the lastest firmware with Knockcode that T-Mobile pushed out.
Best way to discribe it is the phone gets "stuck" on whatever band it falls on. If my LTE is weak, it will fall back on 3G or 2G and just stay there - it seems like networks aren't prioritized. The phone should (in a perfect world) look for 1) LTE, 2) WCDMA, and then 3) GSM in that order and only pass one over if weak or doesn't exist, and should it pick up a higher ranked connection it should switch back. It seems the LG G2's just don't... If it falls back on 2G it's perfectly happy staying there until force it back to something else (default mode LTE/GSM/WCDMA) and it's rather annoying. LTE only is viable only when T-Mobile enables VoLTE on the G2's, but as of right now it stops all calls from happening, and I can't even change my phone to only use LTE/WCDMA, because where I work we just got a tower upgrade to LTE and they took the 3G offline, so calls still won't go through since it has to fall back to 2G.
I'll give Samsung some credit when I had my S3 and S4 on AT&T they would always follow the pecking order of what I stated above (in a perfect world) and I never had an issue, but the G2 blows the S4 out the water in performance in my opinion.