Firmly entrenched in my top tier of Romance, Horimiya is possibly the most unique case ever. Originally done as a 4 panel webcomic then serialised in a magazine.
Usually studios are working on incomplete source material. However in this case Cloverworks was presented with a project from a completed manga and given 12 episodes. Thus, they focused on the most meaningful encounters and cut a lot of the SOL material from the source. The result is going to feel like a speed run as CW chose to get to the ending within the 12 ep limit. S1 will there give you the whole main story.
That was winter of 21. Then comes the next winter season and MDUD posts big commercial success and Kaguya S3 sweeps in right on the heels to post equal numbers. MDUD was only supposed to be a one and done and there isn't enough material to even think about starting production. How are they going to keep riding the new romcom wave? Weeeeell, there's all that Horimiya content littering the cutting room floor. S2 is Named Missing Pieces because it restores those deleted scenes.
I really wish that CW would remaster whole thing back into proper order. I enjoyed Pieces but I spent most of my time watching trying to figure out where we were at.