HC-12 modules stopped receiving, cannot understand why
For what it's worth, I manage to make them working again by playing a bit with the settings of both modules.

Using a simple test sketch I issued some AT commands to both modules; I had already tried setting them to the DEFAULT setup with AT+DEFAULT, but that was fruitless. I decided to left the mode as is (FU3), same for the serial rate (B9600), but I tried switching channel so I set both modules with AT+C002.

That did the trick, maybe was some sort of external interference, or a lack of precision in the channel settings - I just know that's a week now they've been communicating just fine and that's what matters.

So, if they fail to communicate, first try to adjust their settings, don't assume the DEFAULT ones are surely working.