Okay, hopefully I don't confuse anyone more here. If I lose you somewhere along the way just let me know

. Also note this is based on my understanding of the article I linked earlier. I'm no expert on this either
Background info:
Sprints Tri-band devices have only one transmission route for 2 radios: voice/3G or LTE data. Meaning you cannot have both radios communicating with the network at once.
With a fully upgraded tower, this isn’t an issue. Your phone will maintain an LTE connection, and when you receive a call, Sprints system will first ping the device through the normal voice channels, but it of course will not find it because the device is only connected to LTE. The network will then send a signal to your phone over the LTE connection telling it to disconnect from LTE and connect (fallback) to 3g/voice to take the call. This is referred to as circuit switch fallback (CSF).
Now for the less than ideal scenario...On a tower that is not upgraded with the proper equipment to support CSF, if your phone connected to LTE it would not be able to take calls, as the towers wouldn’t be able to relay the information to tell the phone to disconnect. That obviously presents a problem in that if you want to use the phone as a phone you are completely unable to do so. Sprint gets around this problem by having their tri-band devices completely ignore the LTE signal on a nonupgraded tower. You sacrifice the faster LTE data for the ability to place calls. What this means is that if you are in an area without the proper network vision upgrades, the phone will not connect to 4G at all.
My reasoning for why what you are experiencing isn’t a CSF issue:
Basically, if you are connecting to 4G, you are connected to a tower that has had the proper upgrades. If the tower was not properly equipped you would not have ever connected to LTE in the first place. CSF issues cannot be fixed by toggling airplane mode, as the device would just hop back on 3G/voice again. Thats why I previously recommended you stop by a store and see if its connected to LTE, because that alone means the towers in your area (or at least the one its currently connected to) aren't affected.
I live in an area that is not fully upgraded. On a non tri-band device (say the galaxy S3), I can pick up LTE fine. However, on a tri-band device with the same non-upgraded LTE tower the tri-band phone will only connect to 3G/voice services. The problem you are describing where the phone will have issues switching between 3G and 4G happens on a non-triband device as well. This shouldn't be related to CSF. Try updating your PRL and profile.