Multiple things can be true at once...

I think Mafuyu genuinely holds deep affection for both Ue and his previous relationship with Yuki. There’s this quote that I’ve always really loved that’s basically, “grief is love that has nowhere to go.” Ue has been, and will continue to be, a meaningful outlet for Mafuyu to explore his feelings of grief safely, though painfully and not without complications. It’s understandable that everyone involved is struggling through this because some things you just have to struggle through - it’s not a slight to those around you. I think the mangaka does a wonderful job of showcasing these seemingly contradictions. With that said, I highly doubt we’ll ever see Yuki’s last moments from his perspective. I think we’re not meant to know, only infer and sit with so far in as we’re able, just as the rest of the characters have to.