If members are able to speak while the Muted role is on them, that means they have other roles that grant specific permissions to users that lets them talk or speak in channels.
Fixing this isn't as simple as others.
If you want to ensure that members cannot speak in voice and text channels, ensure the other roles that are applied to the member do not have these permissions specifically enabled in those text and voice channels, but make sure the @everyone role in your server still gets the general permission to speak in all of the text and voice channels and not in specific channels.
Or, if the server is set up in a different way (such as a verification method on entrance / other channels that are hidden from the public), configure the channel's permissions to have the send messages permission checked for all the roles (including @everyone role) and deny it for the muted role.
If the problem persists, or another error shows up after doing this, please join the support server.