I am just wonder why the multiselect value were not returned as comma-separated string.
What if the value is itself already space-separated?
This is part of the XLSForm rules and more details are available the documentation on choice names:
The
namecolumn of the choices sheet defines the values that will be saved when each choice is selected during data collection. Choice names for select_multiple must not contain spaces because spaces are used as a separator when an answer with multiple selected choices is saved. Choice names for select_one questions may contain spaces. However, we recommend avoiding them to make analysis easier. Additionally, this makes it possible to convert the question to a select_multiple in a future form version.