What about older raspbian images?
For example, I’m experiencing issues adding Mender to 2018-04 raspbian lite. It works just fine of RaspberryPi 3B, but RaspberryPi 3B+ can’t boot, I’m getting this sequence running from freshly burned image generated with mender-convert:
rainbow screen -> screen with raspberries but without any output for couple of seconds -> reboot > rainbow screen forever.
Where to look for a fix (if updating to a newer raspbian is not an option)? FIY, all newer versions of raspbian lite work fine.