I am not sure this is necessarily a Parsoid issue. I think VE might have dropped the comment during the edit of the parameter or could have been an editor issue where the editor might have unintentionally deleted the comment while updating the parameter.

Right now, Parsoid considers that comment as part of the template parameter and encodes it as such. So, this could be considered a Parsoid issue if the current representation of the comment is considered inadequate for preserving them. I am not convinced that we need to change the representation since I can imagine those changes replacing this "edge case" with other edge cases.

So, I am inclined to classify this as a VE / user-error rather than a Parsoid issue.

This is actually caused by TemplateData for that template parameter specifying it is a single-line text. We've just fixed a duplicate this issue filed as T190191: Line-breaks between list-items in template parameters are removed. :)

Though do note, I think this'll still happen if it's a comment next to any more-specific TemplateData type. I.e. a comment after a boolean is going to get thrown away because the value will get coerced into a checkbox, etc.