Just hope the original slab had a plastic moisture barrier underneath it to keep moisture from traveling up through the slab. Since Ivan came through here last fall, I have been in numerous houses where the tile floors were submerged under 2-3 feet of water with no damage (other than nasty grout). The paint should be OK if it’s still well-adhered. If in doubt, use a fortified thinset like Custom’s Versabond. The only thing is that the mortar may taKe longer to dry if the paint prevents the water from penetrating into the concrete.