That has nothing to do with how the thumbprint/key authorization is being calculated. The TXT RRs value is either good or it isn't when validating. If the value is incorrect, the LE validation servers error message would literally mention and show that.
To me this sounds more like some other issue. I'm assuming you're writing your own ACME client? I'm reckoning you're not triggering the challenge correctly. Can you show the code with regard to the ACME challenge, it's calculation and triggering?