I have seen that on one laptop as well. If I remember correctly the resolution is not found out correctly. You can use a workaround and query modes in the loader(8) prompt by gop list. In my case I used the most likely parameter by gop set 8. That should end up with a proper resolution. boot starts the boot process. To make it permanent after installation add the line to /boot/loader.rc.local or create the file if it does not exist.