Disk Wiper 15 Free - Verification failure

I'm trying to re-purpose some MacBook Pros with SSDs and three of them have a strange problem...

When I boot Disk Wiper 15 Free and try to run SSD Trim, the instant I click the final click to start wiping I get a "verification fail" dialog asking if I want to continue. On two of the systems, clicking "yes" will instantly re-draw the same dialog asking the same question, and no matter how many times I try to continue, I just get that dialog again. On the third system I can continue and (as far as I can see) the drive will get wiped, though the verification fail dialog appears again when the wipe finishes.

Neither Apple's diagnostics nor a S.M.A.R.T. Extended Verify (using another tool) find anything wrong with the SSD. What is Disk Wiper actually verifying?