Whilst you can’t replace the bitumen sealant lining in self sealing tyres, the repair standard is exactly the same as any other tyre.In any case the bitumen lining can't 'officially' be repaired to a standard.
If the puncture is in a repairable area, it can be fixed, just a suitable patch of the seal material has to be scraped back to the rubber of the tyre and the surface prepped. It takes a bit longer and some tyre centres don’t have anybody trained, or just don’t want the faff and would rather just sell you a new tyre.
I had one repaired for £30 at my local ATS tyre place.
This depends on the tyre sealant you’ve used, Holts Tyre Weld or any other water based sealant can be washed out and the tyre repaired as normal. The issue is that a lot of tyre places don’t like to do it because of the mess and the extra time it takes. Warning them the tyre has tyre sealant in it goes a long way to whether they feel inclined to repair it!Fortunately had can of goop (no, you can't repair the tyre after goop, but you can't repair it anyway).
If your local tyre place won’t repair perfectly repairable self seal tyres or ones that have had water based sealant squirted into them, then find another tyre place is my advice.