Ultimaker 3 printing area?
Yes that happens! It finds its source in various possible reasons.

The print bed is not square! There are cutouts caused by the print head;
The Ultimaker 3 has 2 nozzles, when you have 2 active, the effective print bed is smaller than with 1 nozzle active;
Using a brim or a skirt also makes the effective print area smaller.

In re-creating your example, the settings in your projects are not correct (but even if done properly, you will not be able to print it flat on the bed). Take a look at the image below, as can be seen your example still has a brim/skirt reservation, be sure to disable the second extruder (as shown by a cross X before &quot;Generic PLA&quot; at the top).

The only solution to print this on this machine is to print it upright: