The helical made all the difference. Period. End of story.

Helical makes the arrow spin, so it won't be as likely to plane off in only one direction when fronted by a broadhead.

The consistent plane off center with your straight fletched arrows could have been indicating a weak spine condition. Or it could have been indicating a misadjustment in your centershot. If you're really curious, try some stiffer arrows with straight fletch and see.