Acid reflux does not break the fast. It is like vomiting which is not self-induced. Since you have no control over acid reflex, it does not break your fast.

However, if anything comes to your mouth, then you should spit it out.

Allah says, “ Allah does not burden anyone what he or she has no strength to bear.” (Al-Baqarah 2:286)

The Prophet, peace be upon him, said, “Allah excuses my Ummah of actions done out of unwilful mistakes, omissions out of forgetfulness, and that which they do under compulsion, and thus have no control over them.” (Ibn Hibban)

Almighty Allah knows best.