Strata law is quite clear on this – an EC member vacates their seat when they cease to be an owner. The Executive Committee is then required to replace the member with someone who occupies their place until the next election.
Your strata manager really should know this – it’s pretty basic stuff. This is what the law says:
4 Vacation of office of member of executive committee
(1) A person elected as a member of an executive committee vacates office as a member:
(a) if the person was an owner at the time of election and the person ceases to be an owner
(2) When a vacancy occurs in the office of a member of an executive committee … the owners corporation must appoint a person eligible for election as a member to fill the vacancy. Any person so appointed holds office, subject to this clause, for the balance of his or her predecessor’s term of office.
In my experience, the Executive Committee, acting as the representatives of the OC, are required to elect a new member and, if there was no other candidate and you are eligible, you should be invited to fill the vacancy