Not sure which specific flavor you mean. "Tart" can cover a wide range. I just don't know what you mean by "sweetart". Like the candy? I guess I'd have to taste it to know exactly what you meant.
In hoppy beers (commercial ones too), I often get an unpleasant acidity that I assume is coming from the hops. I haven't measured the pH of those beers, but I wonder if sheer hop mass is causing a pH drop. Again, I have no basis for this other than assuming that alpha acids in hops are actually acids that do cause a pH shift if you use enough of them.
I do a lot of late hop addition beers and don't generally have that problem. I also use a lot of the same hops as the OP. I wonder if water chemistry has something to do with it. I use RO water and don't mess with it much.
Check your pH at a few different points and see if it looks odd. I'd be interested in knowing what the pH is of your finished beer that has that flavor. De-gas it, and measure it at 68F.