#fusion360
unwilling to look into it and reproduce the errors (yes, its just one of many pretty bad faults that have been there for years). Which points to a fault deep in fusion360s core programing.
having pulled the part now down further and clicked okay actually works. the gray surface we wanted to fuse it to acutally moved with it.
we measure and for somewhat reason is shows the angle between both furfaces as 179.9973°?! just a glimpse how flawed their snap function is.