Oh, I’m a little bit surpised by that, because 1 stone handicap is basically just komi advantage for black (worth only half a move).
So when there is a full rank difference, white has an advantage when black gets only 1 stone handicap. Around 1d, I’d expect a 57/43 winning chance for white.
I now remember that this came up before: 2020 Rating and rank tweaks and analysis - #97 by gennan. But it seems that OGS was not convinced by arguments made by me and @KillerDucky in that discussion.