There are a lot of reasons for new allergies, and similar reasons for why you might move past them.

Developing a new allergy you did not have prior could be due to some not so fun things, and can also be due to a change in gut flora.

If that change is reverted or changes again, you might not have that allergy anymore.

There is a lot of sense in why our bodies may grow allergic and also get past an allergy. This is considered a widely understood fact too, so if your doctor told you this within the past 10-20 years I'd probably make sure not to see them anymore.