Also in CA. I appreciate you finding this, and including your links to research. My S-Corps are usually more vanilla than this, and none had disposals in CY, but I'm glad to know to watch for this.

I don't have the brainpower at this stage in tax season to dive into this, but my guess is the answer to your "why" question is just that it's a programming error.  So first, heads up that this is only a peer forum.  You need to either raise this to Support by calling or making a Tax Idea Exchange post (if you do, link that post here and I and others will happily upvote it).

What I see/notice in Lacerte is that only CA Form 3804, if I try to do "jump to input" on Part 1 Box 1 (total QNI), only jumps to Screen 42.011 Code 101 (just a checkbox that all shareholders want to elect in).  To fix this, I think there needs to be either an input/override QNI, or linking to include the gain from disposition of 179 assets.  The former is probably an easier ask this of Support late in tax season.