There's simply no feasible way to withdraw the funds on lower end puzzles like #66. It will be snatched up by bots. Not maybe, but it's 100% guaranteed. There will be hundreds of withdrawal transactions with varying fees all battling each other. You will simply be left in the dust.
I don't understand how it is possible for that to happen as long as the solver does not disclose the private key. I mean, #64 and #65 both have unknown keys, right?