I have the same trainner. I wouldn’t believe your Direto is reporting correct power. Mine has been reporting about 30-35w higher consistently than my power2max type s (that has had a stable zero offset since I’ve owned it new). Note I don’t calibrate or do a spin down on my Direto ever minus trying to do it a couple of times when it was new (bought mine in March 2018.). Even then it was reading 5-10w higher than my power meter and the slope on a spin down reported drifted from the factory calibration number printed on the bottom sticker of my trainer.
But, for the ramp test I feel for me it’s a combo of both for erg mode to what you are describing. Once you start to waver cadence and steady power pedaling form then at the higher watts it starts waving up and down trying to compensate. Then the inevitable low cadence death spiral takes place if I let it drop more than 20rpm cadence from what I was doing throughout the ramp sets. Hard to recover for me from that on a ramp test. It’s even worse swings I find in the big chainring. But last 2 ramp tests for me were in the small chainring also.
I also tested out resistance vs erg for the ramp test early on when I first got my Direto thinking I could push a minute or two longer in resistance mode (to get a better test result). Turns out it was pretty much the same result for me as the ftp calculation were within a couple of watts of each other.