THIS.
I had the bolts on my intake do the exact same thing, and I solvent washed the bolts first and sealed the threads and everything.
It didn't stop until I pulled them one at a time and sealed around the heads/washers also.
I used RTV around the bolt shaft right where it meets the washers, put them in and cleaned off any sealant that squeezed out around them with a small stainless steel wire brush. (like a toothbrush size)
Yea, it was a pain in the *** but it stopped that from happening again.
I used an old carpet cleaner the wife wanted to throw out and adapted a 3/8 hose to the suction hose and used that to vacuum out those valleys in the intake while I scrubbed with an old toothbrush and hot soapy water. (while the engine was warmed up)