It was recently discovered that the ACA 7m configuration files for Cycle 4 in the casa simulator use an incorrect primary beam, so 7m simulations may have incorrect fluxes. Simulations of the 12m array are fine.
In order to fix the problem, the first line of the configuration file aca.cycle4.cfg must be changed from "# observatory=ALMA" to "# observatory=ACA".
aca.cycle4.cfg is attached to this knowledge base article.