Adam Aglionby wrote:

L7812 is an amp in a bucket of water, they run hot , its a linear
regulator which means its having to dump 12V from your 24V as heat,
being a clever integrated regulator its probably entering thermal shut
off land.

That's what I first assumed. But it's not the regulator that's shutting
down, it's the boxed PSU for the fan.

Is there something the regulator might do to the supply when thermal
shutdown happens, causing an upstream device to get upset?

Pete