If there was a resignation of an MLA in any local district seat in a proportional system, the byelection would still take place under first-past-the-post - perhaps with a Preferential Ballot. Mathematically, it's impossible to be 'proportional' when there's only ONE seat available!
Under Mixed-Member Proportional, 18 of the 27 seats are 'local districts', so they would have a regular by-election if there were a vacancy. However, if an MLA who held one of the 9 province-wide seats resigned, then 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. If there were no other candidates, then a province-wide by-election would be called for that seat. However, the chances of this happening are very low.
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.)