I think it may very well not be a bug.
I changed also this setting on my 10T Lite and I noticed not all values are acceptable and I've to make by small increments.
For example (all values are made up):
You change custom DPI form stock value 384 to 412. You go back in to check and now it's at 400.
You change it to 412 again and now it's at 404.
Again and now it's at 408. You change it to 420 and it's at 412, and so on..
Subsequently, you change font size.
The system adapts screen's elements sizes and can, even, changes the already set DPI value.
Further more. You change the icon's size and the system resizes them.
Finally, you restart and you may notice a (larger or smaller) adjustment of some (or all) of the tweaked settings