The answer depends on whether it is a district seat or a province-wide proportional seat.
Under Mixed-Member Proportional, 18 of the 27 seats are 'local districts', so they would have a regular by-election (under First Past the Post) if there were a vacancy, the same as now. Mathematically, it's impossible to be 'proportional' when there's only ONE seat available!
However, if an MLA who held one of the 9 province-wide seats resigned, the standard practice in countries that use MMP is that there would be no by-election, and no cost of transition. Instead, the candidate who was that party's 'next most popular' candidate, as determined by the voters in the previous general election, would be offered the seat for the remainder of the term.
In proportional systems, by-elections are likely to be less frequent, because there is more likely to be an incentive for all elected politicians to serve their full term, knowing that by resigning they could affect the balance of power in the legislature. (In the lopsided legislatures common under First Past the Post, the governing party usually has such an excess of seats that many 'backbench' MLAs could resign without it affecting control of the house.)