Is it obligatory to re-fast days that were made up from Ramadan, given that fasting causes GERD and heartburn due to a problem with the cardia, and the condition worsens when lying down?

The fast is invalidated if vomit or regurgitation reaches the mouth and the fasting person intentionally swallows it. However, if it does not reach the mouth, or if it reaches the mouth and is expelled without being swallowed, then the fast is valid. The fasting person must expel whatever reaches their mouth to avoid invalidating the fast. It is also recommended to avoid situations that cause frequent regurgitation to the mouth. As for heartburn that does not result in anything coming out to the mouth, it does not invalidate the fast.