Acid reflux is something that happens involuntarily. A person may feel some acidity or bitter taste in his oesophagus but it does not reach the mouth. In this case it is not regarded as one of the things that invalidate the fast, because it does not come out to the mouth.
But if it comes out to the mouth, then in that case it comes under the ruling on reflux or vomiting.
The word qalas (translated above as reflux) was interpreted as meaning vomiting, or as referring to a small amount of vomit that reaches the throat but does not fill the mouth. And it was said that it was that which is regurgitated by the stomach when it is full.
See al-Majmoo’ by al-Nawawi, 4/4
The ruling on this is that if it is returned to the stomach when it could have been expelled, this breaks the fast, and if a person swallows it because he is unable to expel it, then it does not affect the fast. See question no. 12659.
It says in al-Sharh al-Sagheer (1/700) concerning qalas:
If he is not able to expel it – such as when it does not pass the throat – then he does not have to do anything.
Ibn Hazm said in al-Muhalla (4/225):
Reflux that comes out of the throat does not invalidate the fast, so long as the person does not deliberately swallow it back after it reaches the mouth when he is able to expel it.
Then he said (4/348):
We do not know of any difference of opinion concerning the fact that reflux and blood, which come out from a person and do not go back to the throat, do not invalidate the fast. Even if there were a differing opinion, we would not pay any attention to it, because there is no text that says that this invalidates the fast.
It says in al-Muntaqa Sharh al-Muwatta’ (2/65):
It was narrated from Maalik that he said: Whoever burps and reflux reaches his mouth but he swallows it back down, he does not have to make up that day of the Ramadaan fast. Ibn al-Qaasim said: Maalik said: If it comes out to a place where if he wanted to he could expel it, but then he swallows it back down, then he has to make up that fast. If he suppresses it before that, then he does not have to do anything.
It says in al-Insaaf:
If some vomit or reflux reaches his mouth and he swallows it, then he has broken his fast. This was stated by Imam Ahmad. This ruling applies even if it is a little because it is possible to avoid doing that.
It says in Haashiyat al-‘Adawi (1/448), after mentioning the ruling on reflux:
Reflux is like vomit, which is what is regurgitated by the stomach when it is full.