Very odd boot issues from Pi 3 B and the Pi 3 B+

I have a working image that I have working on the Pi3B, no issues.
Bought the newer B+ for the extra speed. pop the SD in and it won't boot (lightning bolt on the screen and pwr led has 4 slow blinks, 4 fast blinks). This is apparently a Low Voltage protection, which is a good thing.

Here's where it's odd. I can get Raspbian and OSMC to run fine on it. Tried a fresh DL image (18.04.20) and I have the same issue.

I returned it assuming there's something up. Just got a replacement one in hand and I have the exact same results.
I still need to test the voltage (have to wait till I get home as I didn't bring a meter with me today). I've tried 4 different supplies that I have had work on the Pi3B, so I presume the supplies are fine. Quite odd that 2/3 images have no issue though. Any insight here would be greatly appreciated!

Cheers