I haven't 100% figured how this bug is triggered, but I have had some chance to exclude a couple of things... When there are multiple transactions from the vendor in rapid succession the 20M wood that is in the vendor seems to vanish after the 2nd transaction in quick succession, so instead of there being 20M wood available, it drops to 10M with the second transaction zeroing out the remaining wood. I've had a few days to test this with less/no people showing up to the vendor, and limiting buys to once every 'tick' seems to preserve the 20M wood, but I'm not entirely sure if it's purely the rate of purchases, or the quantity in each of the purchases, or both. Needs more testing, but halving the wood supply per day does seem to be leaving some people sans-wood.