I think the issue here may be the difference between how Unscheduled & Scheduled orders work in terms of reserving inventory.
You guys don’t have “Reserve on Pick” (which would only reserve inventory at time of picking) nor do you have “Reserve Inventory for All Orders” set. It sounds currently like you might be expecting the operation of the latter here though.
With your current settings: Unscheduled Orders such as 5156.1 should reserve the balance due of the line components (& subcomponents if BOMs are in play) at time of commit to order.
Scheduled Orders however, are a bit more of a special case. 4551.1 is scheduled, and it’s Start Work Date & Ship Date are far in the future, so, it won’t reserve any of the subcomponents until it’s Start Work Date. (If Leadtimes were in play on those subcomponents, it would subtract that number of days/weeks from the Start Work Date & start reserving the quantity on that date).
So, with your current settings, I would expect to see 18-20 avail from the Build Estimate page. If you’d like to change the config to Reserve Inventory for All Orders, you as an admin of course may do that.
Hope that helps!
Please let us know if you have further questions on the matter.