Is it obligatory to re-pray the prayers that were performed while chemical henna was present on the hands, knowing that it caused skin burns and the formation of a dead layer?

If medical evidence proves that black henna is harmful, then its use is forbidden, based on the Prophet's (peace be upon him) saying: "There should be no harm nor reciprocal harm." However, if it is washed with water and its color remains, this does not prevent water from reaching the skin, and ablution (wudu') and ritual bathing (ghusl (definition)) performed with it are valid, because after its substance is removed, the dye becomes merely a color that does not affect the validity of ablution or ritual bathing.