Sounds to me like the material isn’t exactly flat/level.
Wood, acrylic sheet and most other things you would want to cut (unless they have been machined beforehand) may look really flat to us, but there are generally quite significant variations in thickness and overall flatness. Wood is particularly bad for this, but I commonly see +/-0.5mm variations in thickness in acrylic sheet, so depending on how deep your cut is (default 0.1mm for relief) you could end up doing quite a few passes before you end up actually cutting into the material over the entire size of the job.
Overall, as long as the total depth of the job is a couple of mm or more, it really isn’t going to have much of an effect on your end product, but if you really want to make sure everything is flat, before starting the relief carving you could mill the surface of your material flat. (it’s a bit of work).
- Make a vector image containing a square (inkscape, illustrator, could possibly just download one)
- Import that vector into Luban and scale the square to fully cover the area you wish to do the relief carving in (probably best to go a bit larger)
- Use the flat end mill
- Set path to “Fill”
- Set target depth to 0.5mm
- export and run this job on the material you wish to relief carve
As long as there isn’t variations over 0.5mm in your material, you should end up with a nice flat starting surface and you can just switch back to whatever carving bit / mill you want to use for your relief carving job. As long as you set your job origin and check your boundaries to stay within that new flat surface you’re first pass will end up cutting uniformly across the entire area.