As far as I know, all the (L)GPL does is make sure that if A releases some code under it, then B can't release a non-free enhancement without A's permission. A can still do whatever they want, including sell ownership to B.

Neither GPL nor MIT (or anything else) protects you against this.

(EDIT) scenario: I make a browser extension and release v1 under GPL, it becomes popular and I sell it to an adtech company. They can do whatever they want with v2.