I may have missed the comment about extract + mini-mash. I am really sorry, because my advice would have been different. I assumed all-grain due to the sparge comment.
The extract that you use already has the needed -ions to match its general style (pale, pils, etc.), and it can be a bit dodgy trying to manipulate the water profile. I have yet to see any reports or numbers on what LME or DME contains relative to minerality. I would try again using DI or RO water without any water treatment. Maybe try a 1 gallon batch to test to save the costs.
If you are truly mini-mashing, then that strike/sparge water should be adjusted slightly to hit the proper mash pH, and then the runnings added to the extract brew. If you are just doing a soak, then I would just use the same DI/RO water for that as well.
I am puzzling over your comments. Can you give more detail? Typically, bicarbonate is tasteless, but the high alkalinity can lead to astringency in some instances. Perhaps just an overall minerality? If the extract has oxidized on its way to you, it can have a slightly papery or even metallic. A sharp bitterness could be from the hops or perhaps overly tannic roast and dark crystal malts in the mini-mash and compounded by astringency.