https://www.gamespot.com/articles/destiny-2s-gear-power-changes-at-faction-vendors-e/1100-6453607/

Max-level Destiny 2 players have something new to consider when it comes to gear drops. It has been discovered that the power of gear drops from Faction/Planet vendors changes every 30 minutes, but as mentioned, this is only for people with max-level Power.

As Reddit user LeafBlitz discovered, the Engram level for gear drops changes every minutes at those vendors (but maybe not all of them). What's the significance of this? If you're patient, you can check every 30 minutes to see what the level of your Engram is, and if it hits 300, you'd probably want to cash them all in to reap the benefits.