using screen multi-select choice in formula uses label not value
sorry the images turned out huge and this article is hard to read
if you reference a screen multi-select choice element in a formula, the formula uses the choice label, not the choice value.
choice with value defined

choice is used in screen multiselect checkbox group :

formula field references screen field

output uses choice labels

but, if you first assign the screen field to a variable, and use the variable in the formula, then you get the values.
assign screen field to variable

formula uses variable

output uses choice values





