"but how did this
receiver ever work properly as initially designed"

Having dealt with leakage on PC boards it might be that internal leakage in the IC pulled it to ground but with very high resistance.

Over years and exposure to humidity it picks up other leakage from traces on the PC board and throws off the bias.

Either that or there were high value resistors internal to the IC that have gone open over the years. the higher the value the more likely they are to open up. True of regular resistors, why not ones internal to an IC. I have actually had this on a TV once. Had to add a 1 meg to ground to fix it, or replace the IC. The 1 meg did just fine.