I’ve had similar experiences and have started replacing the cap when working on them.I had a cracked one the other day that for the life of me I couldn't see the crack. I was there a few weeks prior for it running, replaced the innards and it was good. Went back last week for a couple others and they said this one would sometimes flush itself since I had been there last.
I changed the innards again because the diaphragm had some weird clearish/black gouk, only a little though. Town's been workin on the waterlines so not a surprise. I get it all assembled and flush it several times, seems good. Flush it once more for giggles and a couple drops come out of the top. Open it up and there's water above the cap. I inspected everything, reassembled it, cranked the top on a little tougher and again, five or so flushes and it leaked!
I replaced that A-71 top cap and everything was good. I checked it several times in between fixing the other valves. I think it was slowly relieving pressure from above the diaphragm which would then cause it to flush. Still never found that pinhole, flexed it, tried a light, nothing. The bottom edge was smooth so I don't think it was leaking out the rim. With water between the top caps it had to have had a crack.
Got tired of the call back on that 1 in 50 that’s cracked.