Retrieve most recent values in multi-select column using formulas

I am struggling to get the formulas to retrieve a value in a multi-select cell. For example:

Flavor options in multi-select dropdown: Vanilla, Chocolate, Mint

Flavor1: Vanilla  (row 1)

Flavor2: Vanilla, Chocolate  (row 2)

Flavor3; Vanilla, Chocolate, Mint   (row 3)

In a separate column I want to use a formula to retrieve the row number of the last instance of Chocolate (Flavor3). So the output would be "3." In a single-select dropdown list, I have successfully used INDEX / MAX / COLLECT to retrieve the last instance of a value.

But I cannot get the formula to work for a multi-select. I have tried using the new HAS( ) function as well, but it does not accomplish my goal either.

Has anyone else encountered this challenge and have any ideas?

Thanks!