Agree, buy castings from Reaves & all is OK
I can only guess why your brass buttplate cracked. Most foundries add some lead to ensure that they get a sound casting.
So my guess is that the foundry who made your buttplate added more than the usual amount of lead, or the lead ended up mostly in the heel of the plate.
It melted during the anneal, which in itself isn't bad, but then the thing shrunk when quenched and the stress from that quick water cool tore apart the areas where the lead was still molten. That is a guess, as normal practice is to suggest one quench from anneal to keep from having problems with the lead.
One reason you should never heat a piece of brass to bend it is that much brass does have lead in it, and if you try to bend it while that lead is more or less molten, it will crumble.