Relative move affected by home offset
Yes this is expected, with M206 you set a relative offset!


  The values specified are added to the endstop position when the axes are referenced.


reference: G-code wiki M206

So when you issue the command M206 X-10 Y-10 twice (or already have an offset set), the offset is 20, 20 (X, Y) and when instructed to move "nothing" (G1 X0 Y0) the printer moves "nothing" from (thus to!) the newly defined origin at (20, 20). Basically it is relative movement from the set coordinate system, not the position of the carriage. In effect you have changed the reference frame, the printer just responds to that action.