A select_multiple field doesn't actually have a domain. Though you define your choices and labels in Survey123, what is stored in that field is a comma-delimited string: "choice1,choice5,choice2", etc. And if I recall correctly, the order in which those items appear is based on the order they are clicked in the survey.
A domain, on the other hand, is for associating unique values with labels. Given the way the field's contents are stored, you will end up with many more unique values than the number of choices.
What is it you wish to do with the values? You can use an Arcade expression to decode values and pull out matching strings.
- Josh Carlson
Kendall County GIS