This report (in Japanese) about a person from Osaka getting reinfected seems interesting:
大阪府は、先月、新型コロナウイルスへの感染が判明したものの、症状が回復し、陰性が確認されていた大阪市の40代の女性が、再…
www3.nhk.or.jp
- Visited Wuhan in the middle of January
- Tested positive for the virus on January 29th
- Discharged February 1st, tested negative on February 6th
- Throat discomfort and chest pain on February 19th, visited medical institute a few times
- Tested positive (again) on February 26th
Could be that the person got into contact with a carrier again or that the virus was not completely eliminated in the first place. The body might have not produced enough antibodies.