Do you have a Snapdragon based S8+? I'm guessing Yes, since you are marked with Sprint. (I ask because I don't know how fast charging on the Exynos chip based phones works)
Which model PowerCore+? This is where Anker gets confusing... For example, the PowerCore+ 13400mAh supports QC2/3 to re-charge itself, but doesn't output QC, it only outputs the standard 5/2.4A . If that is the one you have, it wont fast charge the S8/S8+. And this PowerCore+ 20100mAh doesn't support QC at all (for input or output). Same with the PowerCore+ Mini which only supports 5v/1A input and output. So they wont Fast Charge the S8/S8+ either.
Where as the PowerCore+ 10050mAh recharges itself at the standard 5v/2A, but DOES Output QC3 (the original model was QC2 when first released). This one SHOULD Fast Charge the S8/S8+ (from experience, I can say mine does). The PowerCore+ 26800 only supports QC3 on a single designated port, so using that port, it should Fast Charge the S8/S8+. (I don't have this battery, so I can't confirm)
With reports of unable to Fast Charge using these QC2/QC3 batteries and chargers... I wonder if Samsung got something messed up in the phones? (I remember having issues with older Motos and my QC2 stuff... where the Motos didn't charge properly but my Samsungs did). Or could it be firmware/rom related from different carriers? Like mine is Verizon and works, and yours is Sprint...
I just switched to Verizon, the S8 is my first ever phone with Verizon. What I'm confused is that all the battery banks that I have I've been able to use the fast charging with my Note 7, S7 edge, Note 5, Note 4, S6 edge+, S6 edge, S6, Note edge.