21 September 2023
It might be helpful to see the base of a bullet as well. Inspection of the base and the driving bands is pretty basic and can help indicate hot or cold alloy or mold. Perhaps these bullets are as large as 45 cal, so could add heat to the mold rather rapidly and over heat the mold. That would be reason to cool the sprue plate. BUT, since the base is not shown, unsure if that conclusion is correct. The steam produced from contacting the closed mold containing the cast bullets would be most like distilled water, leaving the chemicals behind on the sprue plate. There is nothing to torce the steam into the cavities. Remember the old method of leaving the last cast bullet in the cavitiy to prevent rust in the cavity? Sort of the same principal.
Do I see a damaged area on the nose of one of the bullets in the second photo? As if it landed on a hard surface on the ogive? Do I see a crack between the driving bands on one of those bullets? That might be from the bullet sticking in the mold before dropping free. This was not mentioned, but if you are dropping mushy bullets, it can happen.
Can you clarify whether you think you have overheated the mold or the sprue plate? I would not expect the sprue plate to be a problem and more likely the mold would be over heated, and at 730 F just slow the tempo and let the mold cool a bit between casting cycles. The cooling between casting cycles will have a quicker response than changing the temp on the casting furnace. Might be time to watch the clock and let it tell you what you need to do.