expired domain 301s did the same thing to me years back, worked lovely for maybe 6 months then just evaporated. so i know that feeling well
but the one i genuinely thought was stupid and would never work was not pointing anything at the money site at all. build a buffer page, blast that instead, let the juice trickle up. first time someone explained it to me i thought it was a waste of good links, why would you water down your own links on purpose
been doing it about 10 years now and its the only thing on my list that never stopped working. survived every update since then. and i reckon thats exactly why it lasted.. it was never leaning on a loophole in the first place, its just how link equity moves anyway. everything that dies is the stuff exploiting some gap google hadnt closed yet, this wasnt that
still use expired domaisn but as buffers now, not 301s. actually mostly fresh brandale names these days, for the same money you get 3-4 of them and no history to worry about