emmc_appsboot.mbn is different, the rest of the modem files are the same.
Don’t get your hopes up just yet.
I flashed the beta modem files manually to my LineageOS 15.1 for microG, and not only did they not fix the shutdown bug, they also stopped the flashlight workaround from working.
Installing the 18.03.1 modem.zip again restored the former status quo.
So, I think either the fix is not in emmc_appsboot.mbn (but in the updated Fairphone OS and Open OS), or it just doesn’t work this way for my LineageOS 15.1 for microG.