We have wax off solvent stripper, you could test an unlaid tile strip half- do a water test on both sides. Then test one tile laid if the stripped half then takes water but has removed the contaminant , once all tiles stripped,you can then replace the nano seal ( pre- seal) with a suitable porcelain sealer.
Just bumping some of the older popular (sometimes not so popular) threads. Probably wont be current discussion these days but I just need to do it. So just ignore the thread if it's not current for you.