Sketchup for Mac 21.1.298
The Add Detail tool has a tooltip “Select a face, edge, or vertex on the mesh to detail. Hold down CTRL to add detail without offsetting the new vertex.”
CTRL + Left Click on a Mac registers as a right click. So maybe it’s supposed to be CMD + Left Click. Which works, once. Try adding multiple vertex in a row and you end up pulling the vertex on subsequent clicks.
So either, the toggle is not working, or the tooltip needs to be changed on MacOS.