If a script include populates the 'To' field with email addresses delimited with a semicolon, then the email client sees that as one long string which is interpreted as an invalid email address. The email does not pick up on the semicolon to separate the addresses, but treats them as only one recipient, as all the addresses joined together.
This is expected behaviour. The use of semicolons is only a UI choice (MS Outlook) to display separation of email addresses. In the script that auto populates the 'To' address field, use a comma as a delimiter instead of a semicolon. If you supply a comma separated list in the email client template script, the Email client will represent them as semicolons, but will allow typing semicolon if wanted.
In MIME format, a semicolon is not an RFC-compliant email address separator (https://www.ietf.org/rfc/rfc2822.txt Section 3.4). In the backend lists elements are comma-delimited, so when an Email Client showing semicolons in the UI is sent in MIME format, the email headers TO, CC, BCC are converted back to RFC-compliant comma-separated to not get rejected.