Having an issue with index formula

Hello, this is my first post here! I'm struggling with the formula below. I'm trying to create a formula that joins multiple instances of something into a single string. The formula below does what I need it do but breaks when I run a script to update the worksheet (even though the cells referenced here are untouched). It seems like the formula resets to an array formula, but when I click on it and press it enter, it becomes a traditional formula that works without the '}'. Suggestions?

=TEXTJOIN("; ",TRUE, INDEX($R$10:$R$999, IF(A2=$O$10:$O$19, ROW($O$10:$O$19)-ROW($O$10)+1, 900),1,1))