That is very minor and well with in spec for a fully adhered roof. I am an industrial/commercial roofer by trade. You will always get a few minor wrinkles and bubbles in a "glued" down roof. You are allowed so many square inches of bubbles per 100 sf. Just keep an eye on it, if it starts to spread you may have an issue.
There are lots of things that can cause the bubble; moisture in the air, not quite enough glue, too much glue, minor debris on the roof, a minor chemical spot on the membrane, etc.