Answered by Shaykh Yusuf Weltch
I suffer from Irritable bowel syndrome, so I get a lot of burps and reflux every day. Recently during fasting after fajr salah, I felt pressure/air rising up to my throat like I was going to burp, but I did not burp, so I intentionally tried to get the burp out.
In doing so, some food came to my mouth, and I felt like I unintentionally swallowed some of it and spat out whatever I could.
In the Name of Allah, the Most Merciful and Compassionate.
If vomit/regurgitation comes up into the mouth unintentionally and is then swallowed, the fast remains intact whether the vomit was a full mouth or less. However, if one intentionally brings up a mouthful vomit/regurgitation and then swallows it, the fast is invalidated. [Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah]
Abu Huraira (Allah be pleased with him) narrates that the Messenger of Allah (Allah bless him and give him peace) said, “There is no makeup on he who vomits unintentionally, and whoever makes themselves vomit must make up the fast.” [Abu Dawud; Tirmidhi]
Hope this helps
Allah knows best
[Shaykh] Yusuf Weltch