I had similar complain from downstair neighbor about leaking in toilet.

Upon checking realized those old tile grout were cracked and some were missing along the toilet floor and wall.

Managed to resolved by resealing all the corners and tile gaps in toilet with silicone sealant.

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The leak point was observed at the joint between the toilet bowl and floor. Especially after cleaning the entire toilet.

Instead of spending effort and money to check and repair what is the main cause, which could be any. I opt for cheaper option by sealing all floor corners and tile gaps with silicone sealant. Two tubes of sealant and one day work. That's all.