Acetone dries with no residue. I use it to prep everything as my last step. Never had a problem.

I would scuff that bracket up to let the 5200 stick. Personally I would use 4200.

+2 on 4200

I could see product being bad, however the internal backing plates were prepped same time as the brackets, and they are polished on both sides.
The only 5200 on those is what got pushed thru the holes and got squeezed out during the torque sequence.
They are not budging and will have to stay put during hole clean out and re install.

Prep/residue is looking more like the culprit, the polished stuff is on good, but the cast/ground surface did not adhere,
It doesnt sound right but the polished does clean real good, maybe the pourus side of the brackets sucked oil up.

4200 is a good option, it was used several places as a sealant on items that can or might be moved since its labeled as removable.
I'll put on anything by any method that will save or prolong the boats life.

Thanks guys

some info

1. cleaning and spotless prep work for both sides of what is being sealed / bonded together -- many in the industry use a cleaner of some sort then finish off using Denature Alcohol and lint free rags.

2. How old was the tube of 3M - I seen some old dates on the shelf from hardware stores and from Home Depot.

3. apply the sealant in a good amount but ONLY semi snug down the bolts / nuts or screws. Let it almost dry but not 100 percent - then tighten all hardware 100 percent. Doing this procedure prevents the sealant from squeezing completely out. IMO this looks like what has happened to you also not using enough sealant applied and the little that was there got squeezed to the outside edge from tightening down before the sealant even set up..

4.Next thru the industry there has been reports of 3 M sealants having issues. Not the mass but it was reported there is / was issues with the product not holding up. This could be the case here as well.

I have installed many keel guards - the best results was from proper prep work and cleaning with a strong cleaner then followed up with denature Alcohol. The keel guards use a peel and stick backing inwhich is 3 M adhesive

IMHO you were lucky that it didn't bond . Like I said in the other thread that's what your bolts are supposed to be for , not the goop . I've seen 5200 bond two pieces of glass together so as has been already suggested , maybe it was defective product
Again , another example of a use for 5200 that I personally would not do.

it has a shelf life in an unopen tube or cartridge. If laid down bead of the 5200 on the ground and let it sit there in the sun - it cures. There is tubes called fast cure 4200 and 5200 - it cures regardless in the air.