I am having a similar issue that is causing quite a lot of lost sales. I have a category discount applied to a top level category. The sales prices all "appear" ok, but when you "Add to Cart" the original price is showing in the cart and Mini-cart. Going to Admin>Calc Sales Prices will resolve the issue temporarily. There is no way to tell that this is an issue until someone finally uses the Contact Us link to let us know, but in the meantime, we loose several other customer sales. Probably only 1 out of 20 actually report the issue. the others just leave.
Running 2.3.4 in DNN 6.1.4
Running 2.3.4 in DNN 6.1.4