Stranded inventory is an inventory at a fulfilment center that does not have an active offer on Amazon.
Inventory can become stranded because of a variety of reasons. For example, the listing for that product may have been suppressed or may have never been created, resulting in stored inventory that is accumulating storage fees but that cannot be sold because the listing is not active.
So yes, Amazon still charges storage fees even if your inventory is stranded.
What happens to stranded inventory?
What is stranded inventory?