The master writs that are dropping during the event already have the writ values in their descriptions doubled, so you will not get double that again when you turn them in. Master writs generated before the event do not get any increase in vouchers at all.
If you are getting fewer vouchers than you see printed on the writ at turn-in, that sounds like an issue. I would also suggest filing a /bug report in game, since the devs have said they prefer that method of reporting and hopefully they can look into it.