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Old Mod
It sounds like it's just the grout powder that got sealed over??
Yeah sure sounds like it could be, DO NOT use any acid based product, it WILL degrade the stone.
Check the literature again and see what it suggests to clean it.
I would firstly remove any heavy residue with the blade.
If it can be cleaned in the conventional manner of using an alkali, pour neat strong bleach on the blemish and cover with a clear piece of plastic sheet. Leave for 24hrs, it will act as a poultice.
Obviously best case scenario would be to try it on a scrap or inconspicuous area first.
Don’t be impatient, let it do it’s stuff.
Then rinse with fresh water.
This will remove any sealant.
Wait a good 7 days before applying any more sealant.
Then seal periodically when staining becomes ore difficult to remove.
Good luck.