I don’t know if that’s a rule of thumb or not, but it certainly makes sense.
First, the world of reliability runs on data and math. Lots of statistics, of course.
And second, aircraft are over-engineered for safety margins on top of safety margins. The test data might say you need a part that’s X thickness of aluminum in order to be 99% sure to never fail in the field. So let’s just make it 3X thickness to be safe!
So from that standpoint, back to back failures should pretty much always draw a bunch of attention in this industry.