As a legacy filters.text has a check that the message does not start with /. In order to not conflict with Commandhandler
https://github.com/python-telegram-bot/python-telegram-bot/blob/master/telegram/ext/filters.py#L246
This is no longer needed because Commandhandler now uses entities to check for a command. In any case this bahavior is not documented.
A user can well be confused by MessageHandler(Filters.text, callback) will not respond to a message that starts with a /. At least the docs should be fixed, but I prefer it to remove the check and just pass any message with text to the user.