TL;DR: Shaxx rewards are 300 for 300 BPL guardians all week, probably
If your guardian has a base power level of 300—meaning equipping your best possible gear would bring you to light level 300 without including the mod bonuses on your gear—vendor engrams that you get from turning in tokens will occasionally drop 300 level engrams for you.
They actually rotate every 30 minutes in most cases, so maybe at 9:00 am Asher Mir is giving 300 engrams for tokens turned in, then at 9:30 it switches up and maybe Sloane is for the next half hour, or maybe no one is.
Lord Shaxx and Banshee-44 don’t seem to be part of this random rotation every 30 minutes, they seem to only change up on weekly reset, and so far had never dropped 300.
But this morning Shaxx started dropping 300s, and because he changes weekly, it seems likely he will continue to give 300 engrams until next Tuesday’s reset.
So if you’re late game and have a guardian that’s at 300 BPL but still looking for some infusion fodder to up the levels of your preferred equipment, you might want to spend a bit more time in the Crucible this week. Or if you’ve got a guardian that’s almost to 300, try to knock out all the other weekly milestones you can to see if you can hit 300 before redeeming any Crucible tokens with Shaxx.
And if you want to find out who else is dropping 300 level loot every half hour, check https://vendorengrams.xyz. It’s crowd-sourced, so the info isn’t 100% accurate or updated immediately, but helpful if you’re trying to complete a specific planetary armor set and would like it to be 300.