Wheel speed speed vs wattage

I have noticed that it is easier to maintain my FTP wattage at 20km/h than at say 13Km/h, is this normal, if i have my cadence at 85 playing with resistance on my Kickr snap, its almost like the tyre doesn’t grip at a lower wheel speed.

For me this changes the dynamic of my session and its noticeably around my FTP wattage. I haven’t noticed this on my direct drive unit

My real question is does speed affect the ability to produce wattage other than cadence which would be a factor when you are out on the road. so would 11km/h producing 250w feel different affect the body differently to 250w at 25km/h

I find I can maintain a high power on a hill better than the flat as you have to keep the power down to keep going where as on the flat you could coast subconsciously and momentum keeps you going - that is why lots of people do V02/Super threshold reps on a hill if they have a long climb in their area - I don’t. That said since I ride tt then actually maintaining power on the flat is really important so I try to train for that.