The lead time being used is not the one I set

Lead time comes from the supplier linked to that variant. Where no supplier is linked, the app falls back to your store-wide default lead time, which is why an unlinked product can quietly forecast on a different number from its neighbours. Where a variant is linked to several suppliers, the app uses the cheapest one, so the lead time you see belongs to that supplier rather than to whichever you think of as primary.