I generally don’t enter Chords on the Chord track by playing them. My templates all have Chord Tracks with an X Chord on each bar which I then edit as needed. But I was able to replicate your problem and what I suspected is indeed occurring.
Attached are 3 screen grabs showing what is occurring over time. These are zoomed in quite a bit so the small timing differences are visible. I just played some random notes and let Cubase make of it what it wanted. Both the Chord Track and the Instrument Track (shown in the Key Ed) were recorded simultaneously.
In this you can see that the first note played is D4. But Cubase needs at least three notes to determine a chord so this alone cannot create a chord. Next in is A#3 which gives us 2 notes still not enough. But the next note A2 is our 3rd note so Cubase can make a Chord event and Cubase decides it is a Bb/A.
Here the next note is G3 which turns the previous triad into Bb6/A so a 2nd Chord Event is created for Bb6/A.
Next note played is a D3 and since there is already a D note in the current chord no new Chord event is created. But the E3 that follows once again changes the chord to Asus4/7/b9 which again causes a Chord Event to be created.