Yes, I see exactly what's happening. You didn't mention that you were using the Crop/Zoom/Pan tool, and it limits how close you can place keyframes together.
Depending on the length of the clip, the spacing can be as close as a few frames in a short clip to the 18 seconds as shown for your 21 minute clip. The reason it does this is to allow for enough time in the finished video for the auto-generated motion arcs and other mid-point keyframes between two adjacent points you've set.
Using the PiP Designer or the Keyframes tool would solve the issue as you can place as many keyframes as close as you like, but you'd then also need to manually handle all the transitions, like using ease-in/out to ramp up/down the speed of any pans if you didn't want the default linear start/stop that standard keyframes provide.
If you want to us the C/Z/P tool for finer keyframe placement, you can cut a long clip into smaller sections. For instance, I show a "no keyframe" duration of just under 7 seconds on an 11.5 min clip.
The downside to doing this is that you may end up at one zoom level or panned position at the end of a section that can be difficult to exactly match when the next section starts, so planning ahead is critical. Will you need to be zooming/panning throught the entire clip or are there sections you'll be cutting out or that lend themselves to a clear opportunity to reset the C/Z/P settings?