When using BOM lines together with Planning optimization the BOM line items are not reserved with Sales orders. The functionality works with the Legacy Master planning which is deprecated. This feature is coming at some point in the future to Planning optimization.

Each possible Item reservation setting which would reserve BOM items is not supported with Planning optimization, for example Item model group.

With BOM lines in use, items are basically always free in stock even though transactions towards the product master has been made. This makes it impossible to sell the product master and have some parts (BOM lines) in stock.

https://learn.microsoft.com/en-us/dynamics365/supply-chain/master-planning/planning-optimization/planning-optimization-fit-analysis