Thanks for having a look. Just as a check, your posts look autogenerated and all in the same format are you human ? I do not mean to offend.
I have checked these points, however these refer to the general grid, and this thread I believe is about the Modeling (and CubeGrid in particular) mode grid which is AFAIK a separate instance with its own settings.
For reference in case it is a GPU issue, I have an RX 7800XT with up to date drivers (24.1.1 at the time of writing). This bug confirmed not to happen on UE 5.1.1 on the same machine.
For OP: Are you following Evans Bohl’s videos per chance ?
I could continue on the tutorial by doing everything in perspective. You could also switch to 4-viewport mode by clicking on the button at the top right corner of your viewport, select your tiles in Perspective mode and “extrude” by watching the orthographic views.