Thinking out loud...
To state the obvious, commodity prices will fall, in general, when the economy is slowing or going negative. The competing factors tending to raise prices are inflation, currency devaluation, and the desire for people to invest in physical things when other investing mechanisms aren't trusted.
The problem you sense is that people won't necessarily pull straight out of other investments and put them into commodities, so for a while commodity prices can go down (investors will pull out of even them) even as investors pull out of other markets. Where does the money go? That's the trillion dollar question. In some cases the money (which is made of thin air) will just disappear as values go down. Some of it converted to FRNs or FRN equivalents. But the balance will be put somewhere. Where will value actually transferred to; where will it move? I wish I could predict that.
Not all commodities will be created equal. If the economy tanks far enough, some commodities may not be worth much at all, or anything. Whereas other commodities will eventually become more desirable and after a time will go up. There is also "money" to be made during the in-between, but that has to be a time of great caution. I wish I knew which thing to buy when. I don't know either, so I'm being conservative.
I am mentally (and physically, by hoarding other things as well as PMs) preparing myself for a possible time (hopefully not seen, or brief) when my physical PM will be worth much less in FRNs. But even if that happens I believe PMs (and some other commodities) will then recover and skyrocket in value.
What is the end point? That's the most important thing. Regardless of all the in-between possibilities that have endless permutations, in the end fiat currency will become worthless and only physical things will retain value. Not all physical things will be of equal value, and a major short-term issue will be that not all things will be easily convertible to other things.