Yes, Shopify. For REAL. This is completely obtuse and absurd. I ran into oversell issues of my own today for this very reason. What on EARTH is the point of having a "Reserve Items" feature on draft invoicing when it doesn't actually deduct the stock from inventory? Like a couple others had mentioned in this thread previously, the shop will instead pretend like the item is in stock on the product page, but then display a "SOLD OUT" error during checkout.
Earlier today, I had 4 or 5 customers get in touch with me about this problem and I ended up manually invoicing them for the technically "out-of-stock" items, which the store let me sell because quantity was still showing from what turned out to be "reserved" inventory. Now, I'm oversold on two products and it's very frustrating to have to contact customers and tell them that I don't have what they wanted after they have already paid. Again, what the heck is the point of having this feature at all when it doesn't actually deduct from inventory? This is beyond frustrating, guys.