Funny you mention this. I was about to create a post for this and saw your post. I have the exact same behavior on my new 12 Pro Max received yesterday. The Apple logo is blindingly bright upon boot up and then vanishes, and then the phone starts up as normal. Have you contacted Apple about this?
*Edit- called Apple support and they are not helpful. The explanation is that OLED gets close to max brightness on boot up. Told him every iPhone I've ever had since the 4 has not done this, including my regular 12 Pro. Either the rep is just dumb or refuses to listen, or a combination.
I tried doing a DFU restore and the issue still exists. I haven't really seen anyone mention this issue anywhere by Googling, etc., this is such a random thing. I hope that there isn't a power surge at boot up causing the brightness to be at max, which could damage the phone.