I think the issue here is 'may contain' is not legally binding in any manner therefore stricken from consideration in any product liability. Your legal liability is 'contains' or 'does not contain', but you cannot list a 'contains' if your product does not contain said ingredient, so now you're legally incentivised to add the allergen to the product to reduce your total liability.
That "may contain" part is what's now being cracked down upon, apparently.
The old law let you say that, and you could sell your stuff as mostly-allergen free but if some got it no biggie.
Now you can't do that, it either has or has not, there is no may. And you can't say it has sesame without it being in there, so adding it is easy.
Otherwise you have to not list it and you have to prove you've prevented cross-contamination.
Most of the time it's not an ingredient but worked on shared equipment. Other times it's like an oil that changes depending on availability.