I have a couple of single frequency systems in my area that actually are MotoTRBO Capacity Plus trunked.
Might I suggest this. Program the frequency in as a conventional frequency. When it stops on that frequency, put it on a channel hold and look at the top of the screen just below the date.
If it shows DMR, it should be properly set up as a single channel trunk. If it is CAP+, then it is a capacity plus, and the data you're hearing is the "rest" channel. Program it as a MotoTRBO trunked system. Even though its one frequency, if it is MotoTRBO trunked you'll want to run LCN finder, since I have seen a few that put it on an LCN other than '1'.
A number of the local schools have trunking on their license, but when I went to discover them, they aren't using trunking - and this is how I validated that.