If I understand properly, he tried to unbind his thread. So it maybe regenerated instantly telling him that fate is unavoidable and him thus feeling defeated by fate (which would explain why he's dating yahiko).
(But at the end of the chapter where he unbinds, the thread seemingly just disapears.)
Then we shift perspective and we follow Yuka. Right after Fuji unbinds his thread, (and it seemingly disappears for him), the red thread appears for Yuka, but tells a different story : this time Yuka and Fuji are indeed soulmates.
The perspective shift obviously making it difficult to guess what's going on in Fuji's head (since we don't know whether the red thread reappeared) , but also showing us that she wasn't in love with him like he was with her, and that she is now, but seemingly only because of that red thread.
- the red thread makes them both act opposite of how they really seem to feel at the start : he was in love but was put away by the red thread, she wasn't but is drawn to him because of the red thread.
- either the red thread lies (or the charachter's perception of it is flawed), either fate can be changed, but in both cases the red thread either isn't fate, either fate is changeable, so good news for Fuji.