Judging by VGT's pixel count data, Ubisoft's dynamic resolution setting for FC6 looks something like this on Xbox Series X | S:
XSX:
Target resolution: 3840x2160
Target frame rate: 60
Max res value: 100%
Min res value: 80%
80% = 3072x1728
XSS:
Target resolution: 2560x1440
Target frame rate: 60
Max res value 100%
Min res value: 75%
75% = 1920x1080
The Series S gets a bit more aggressive dynamic res setting along with cutbacks to draw distances, so it makes sense that it has the most stable 60fps in comparison, which is great.
For PS5 though, it's kinda odd...
PS5:
Target resolution: 3392x1908
Target frame rate: 60
Max res value: 100%
Min res value: 87%
87% = 2944x1656
Unless the PS5's max target res is also 2160p and VGT failed to notice, the min res value based on VGT's data is 87%. So under heavy GPU load conditions, PS5 gets less wiggle room to adjust its res, hence there's more tearing, less consistent 60fps than both the versions.