I agree, very odd. I have a vague memory that this started when I switched from Lilo to Grub? I suspect it has something to do with setting screen resolution. Could be a glich with Menu-1st settings?
If I boot without the external monitor connected - it's just plugged in directly so I just unplu it - then all video output is visible on the laptop screen.
I have notice that requesting detailed boot information from the system splash screen (in Grub or in Lilo) results in the external monitor video output during boot being legible.