The firmware files that we distribute might be too old to support the 3A+.
If you are willing to do some fiddling around you can try the following steps to test out support for the A+ (because we don’t have one here to test with).
Boot the image on a Pi 3B+
sudo apt install dphys-swapfile
Install deb building packages:
sudo apt install devscripts dpkg-dev
dpkg-source -x raspberrypi-firmware_1.20190215-1.dsc
cd raspberrypi-firmware # Check to make sure that this is correct
dpkg-buildpackage -rfakeroot -b
dkpg -i ../*.deb
Then reboot the 3B+, to make sure that it still boots, then put the SD card in a 3A+ and it should work.
If that works, then this will be a part of the next image allowing it to work out of the box on a 3A+.