Slitherine

During the AO DLCs your prestige can be resetted to half or to a fixed amount if you choose to trade it for the rewards.

An easy way to store prestige, before such trade happens, is to acquire new units and place them in reserve. Once prestige is traded disband the units placed in reserve and get the prestige back.

Devs know this trick, and they have no plan to "punish" those who do this. Let everyone do what they want to their own campaign.

But why?

You're more likely than not swimming in prestige.

It's a thing that will likely never get fixed since it does not really needs to be. As nexusno2000 pointed out, the amount of prestige a player gets in a campaign is more than likely going to cover the needs to get all the units ever needed. If a player is good enough to accumulate large amounts of it, he will always be able to do so later, even after a prestige reset, so that ability to stock it in "spare" equipment is not really relevant. For anyone still learning the ropes and not having that much this will also not make a difference since he won't be able to store anything major. So abusing this method is a really marginal thing that will rarely come up and not break the game balance in any way.