FormPart can only hold one value for each form parameter. This is partly a bug in the parser (here and here) but it is also a bug in the FormPart's type definition:

FormPart* = object
    data*: OrderedTableRef[string, tuple[params: StringTableRef, body: string]]

Since the body field is string instead of seq[string], Prologue cannot properly handle data from a form input like this:

<select name="thing" multiple>
  <option value="Thing 1">Thing 1</option>
  <option value="Thing 2">Thing 2</option>
  <option value="Thing 3">Thing 3</option>
  <option value="Thing 4">Thing 4</option>
</select>

This is a pretty trivial thing to fix, but it does require API changes.