Zeng, 19, tested positive for acetazolamide from an out-of-competition urine sample collected on October 18, 2018. Acetazolamide is a Specified Substance in the class of Diuretics and Masking Agents and prohibited at all times under the USADA Protocol for Olympic and Paralympic Movement Testing, the United States Olympic Committee National Anti-Doping Policies, and the International Gymnastics Federation Anti-Doping Rules, all of which have adopted the World Anti-Doping Code and the World Anti-Doping Agency Prohibited List.

Provided evidence
During a thorough investigation into the circumstances of her positive test, Zeng provided evidence, including whereabouts and prescription details, that the prohibited substance detected in her sample was from an altitude sickness medication prescribed to her parent that was given to her in response to an illness she experienced while traveling in a high-altitude location. Zeng indicated she believed she was being provided ibuprofen and, therefore, did not have or apply for a Therapeutic Use Exemption, which is required to authorize the use of a prohibited substance in sport.
The suspensions in cases involving Acetazolamide ranges from public warning to two year suspensions. Most athletes has gotten one year suspensions or longer.

Not very common
Zeng is one of 39 athletes in the sport of gymnastics who has been suspended for an anti-doping rule violation (ADRV). She is also the sixth gymnastics athlete from the US who has returned a positive test for a banned substance. Russia has just as many doping cases.
In gymnastics Furosemide is the most commonly used substance. The Anti-Doping Database has registered 12 cases involving this substance.
18 athletes has tested positive for the Acetazolamide. Track and Field has the most cases with 5, followed by cycling (3) and swimming (2).

About Acetazolamide
Acetazolamide is used in the treatment of glaucoma, drug-induced edema, heart failure-induced edema, epilepsy and in reducing intraocular pressure after surgery. It has also been used in the treatment of altitude sickness, , increased intracranial pressure and neuromuscular disorders.
The substance reduces the activity of a protein in your body called carbonic anhydrase. Blocking this protein can help reduce the build-up of certain fluids in the body. It is used in people with certain types of glaucoma to reduce the amount of fluid in the eye, which decreases pressure inside the eye.
Acetazolamide is also used as a diuretic ("water pill") in people with congestive heart failure, to reduce the build-up of fluid in the body. This build-up is called edema. (Source: Wikipedia and Drugs.com)