As of the latest hotfix the Tauforged Topaz Archon Shards do not give +1.5% secondary critical buff per kill on an enemy affected by heat according to the in-game UI.
How to reproduce:
-Kill any 1 enemy affected by heat status, buff is not shown in the UI.
-Killing 2 enemies sometimes gives 4% and sometimes give no stacks.
-Inconsistent.
Note: I am using 3x of those shards on my Mesa. So the 4% value gained is with 3 of them equipped.
In survival, it took about 400 kills on enemies affected with Heat to give the full 225% buff, instead of just 50 kills.
Also, for some reason the max buff shown on the UI is 149% with 2x tauforged shards and 224% with 3 of them.