When passing multiple email addresses joined by a semicolon to the To, Cc or Bcc fields of the Compose Email action, the generated mailto link will omit those fields entirely. However, mailto does work with semicolons and most email clients will parse those correctly as multiple addresses.
Currently I can work around this using a Rich Text component and a Template column, but this can easily become quite complicated when some fields are optional. For reference you can use the template below:
For those interested, I created an experimental column on Repl to generate a mailto link with support for all legal characters (including semi-colons).