In my experience playing ESO with a PC on the NA megaserver, the Master Crafting Writ tooltip that is displayed while the Writ remains in the reward container still continues to declare the reward as 0 vouchers. If the player chooses "Goodbye" to the offer to immediately convert the Writ to a Master Crafting Quest, then the Writ is stored in the PC's bags.

Thereafter the Writ tooltip discloses the correct number of vouchers that will be paid after the Quest is accepted, completed, and the item delivered to NPC Hollis Hlaalu at the appropriate location. So, the only consequence of the oversight is that the player must remove the Writ from the reward container to learn the voucher value. (In my humble opinion, ZOS has no incentive to remedy this oversight.)

Note also that the number of vouchers that the PC will receive is not disclosed after the Writ is converted to a Crafting Quest. The reward for completing any quest is never disclosed until the PC completes it and contacts the NPC that will reward the PC.