This is more an annoyance than a bug that needs immediate attention, however the Calamares installer doesn't actually *check* in a BIOS GPT setup to see if the conditions are met, it simply complaints and gives the notice you need an 8MB unformatted partition marked bios_grub for the thing to use.
This being said, it's very annoying.
(1): Manual GPT partitioning, selecting the conditions for the bios_grub partition:
(2): The completed manual partitioning schema:
(3): The message which pops up
The installer should be able to determine what was provided (see the first and second screenshot) and determine whether or not to actually DISPLAY the notice in the 3rd or not. If the conditions are met (or exceeded, if I give a larger than 8MB partition, say a 10MB partition or such) then it should not give me that prompt.