I have the same problem at times with Cortana.
Thing is, I can accurately predict exactly when and where it will happen, and I can duplicate the issue on purpose.
I'll tell you what it is that triggers her "mental instability" as I call it. At least I'll share what triggers it for ME.
When I'm driving down a particular road, and I get to a certain point in the road, there is a momentary lapse in internet service of about 2 seconds or less if I'm driving the speed limit (and I generally do.) I've been able to locate the exact location by stopping at numerous points along the route. It's a back road in the middle of nowhere, so stopping wasn't an issue as long as I had time. If I try to give a command while passing over this area, where there is a momentary "disconnect" with the outside world while issuing a command to Cortana, she will mess up what she thinks I said every time.
There is another area in town, where, if I'm standing still, my phone will occasionally and randomly switch between two towers that have the same signal strength. If I'm giving a command to Cortana when it switches towers, she messes it up.
If I am in an area where I have signal, but the signal is low and the internet is kind of bad, then Cortana also messes up the commands.
If, however, I am on a strong signal with LTE, she never misses a word I say. Also, if I'm on a strong and fast Wi-Fi signal, she never misses a beat.
Since she is based in the cloud, I think the connectivity has a lot to do with it.
If you've confirmed you have a good signal or strong and fast Wi-Fi, then I have no idea what could be causing the issue, though a soft reset (power off and power on) might help with it.
Let me know how that goes.