If the adhesive from the braces or the aligner attachments was not properly cleaned off the surface of the enamel, over time it can pick up stains and become visible as a brownish spot on the tooth.
Another possible cause of staining is white spots that developed during treatment as a result of poor oral hygiene. White spots are caused by demineralisation of the enamel, and if they are left untreated, they darken over time. This happens because of colourings in food and drink (coffee, tea, wine, spices) and the progressive breakdown of the enamel, which makes these spots more noticeable.
To prevent these problems, it is important that after braces or attachments are removed, the teeth are professionally cleaned and, if necessary, remineralisation therapy is carried out to restore the enamel.
This answer is for general information. Your own case needs an examination by an orthodontist.