What you are doing sounds funny. Maybe an installation error or some kind of glitch. Something to contact the makers of the software but good luck with that.
I find that one of the easiest ways to define a board shape is to use another layer as the outline of the board. I use the Keep-Out layer because it is also used as the board outline for the PCB manufacturers. Using the Keep-Out layer, draw a bunch of interconnecting lines in the shape of your board. Make sure that they all connect. Press Shift+S to highlight only the layer in use. Drag a box around everything to select everything in that layer. Then from the Board Shape menu, select 'Define from selected objects' and voila, your board is now defined.
Yes this does seem to be messed up. Mine behaves like you describe and it may be not unique to your install. To add to the prior response, I typically define the board shape on another mechanical layer (e.g. layer 20) and I can add in dimensions on a corresponding mechanical layer (e.g. layer 21). Often these have been imported from a DXF file from a mechanical designer and then I modify them as needed to add pull-backs or edge cutouts and such. Then I select the outline and copy it to the board outline layer and set the line width to be very thin. Then I can define the board shape from the outline.
After that I can edit the lines on the board outline layer as needed and redefine the board shape as needed. The board outline then gets copied to the keepout layer and the lines made thicker to define the board copper pull-back from the edge.
I have a completed board and I noticed that when I view Board Information it says one of the sides is off by just a tiny bit. So, I went to click Edit Board Shape thinking I could click on the lines and have it snap into place or possibly even manually enter the coordinates. However, when I click on Edit Board Shape I can't click anything. In fact it seems that I get stuck in that command and the only way out of it is to close the file and re-open. Am I doing something wrong?