That's normal behavior for all of the tri-band Sprint Spark devices. The phone only has a single radio path and uses CSFB (or eCSFB depending on the Network Vision market you're in) to basically "check in" to the CDMA network every once in awhile to check for texts and incoming calls while you're on LTE.
Your phone isn't capable of SV-LTE (simultaneous voice / LTE) which means when a call is happening, the single radio in your phone is being utilized on the CDMA network to make that call, not being connected to LTE. LTE only carries data, not voice. So just like iPhones, which you mentioned, when you're in a call, you are not connected to LTE. When the call ends, it pops right back on the LTE network. You can get simultaneous data on your phone if you are connected to a WiFi network while on a call, though.
This configuration in phones is what allows Sprint to have all three of their LTE frequencies supported on the phone (800mhz, 1900mhz, and 2.5ghz) and has the added bonus of having better battery life than phones with multiple frequencies being connected all the time.
There were some initial issues with CSFB in some markets for Sprint when the tri-band devices were rolled out, but those have been fixed to my knowledge.
If you have additional questions on how your phone works or what's happening with the Sprint Spark or Network Vision rollouts, visit
S4GRU.com. The people there are very knowledgeable and helpful.