After some more testing, ive noticed that its not that you cant trigger either item but under some conditions you can and others you cant.
If you first buy Quicksilver Reload and then push the damage threshold of Crimson Slash over 80 (trigger for Mystic Burst) then burst wont trigger most of the time. But if you first push the damage over 80 at a base and then add Quicksilver Reload on top it works.
If you get Burst first and then Quicksilver Reload after that it does work consistently but only if you exclude the damage from Quicksilver Reload.
This makes me think Quicksilver Reload 50 damage is not calculated consistently for Mythic Burst to trigger, and only under some conditions does it trigger while including the 50 damage in the calculations.
Not sure if the 50 damage is even intended to be calculated in the damage to trigger Mystic Burst, but sometimes it does most of the time it does not.