RE: Alternative item in BOM/formula
Hi Henrik,
I do agree that quantity for alternate item does not look right, but, after retesting by adding on-hand for main ingredient, I found that, in this case, the required quantity for main ingredient is the expected one (see below screenshot).

More, I have tested same scenario on latest D365 release with same results (i.e. when on-hand exists only for alternate item, required quantity for alternate item is a sum of both required items, but when we have on-hand for main item, then required quantity is the expected one being set on formula).
Cannot say at this point whether the fact of the the 2 quantities are being summed up is correct, but with reference to your scenario, you might want to adjust the quantity for alternate item in such way that, when on-hand exists for alternate item, the sum will be the expected quantity for alternate item.
Also, you might want to raise this issue to Support in case you have the time and possibility.
I hope above will help.
Best regards,
Gabriel