Ah yes, Topaz shards are still bugged. And no idea how they are supposed to work either. Note that I'm talking about critical chance for secondaries only here, maybe other buffs (maybe from other shards as well) are bugged too.
First and foremost: heat damage ≠ heat status effect. On shard description it says 'heat damage', whereas on UI icon it's actually the latter. And it seems that UI icon is the correct one (at least for now), because you gain precisely zero stacks by killing enemies with heat damage. They need to be affected by heat status (whether they die from direct hit or from DoT doesn't matter as long as they have heat status on them when they die).
Secondly, you can't gain new stacks by killing enemies in succession. I've tested it several times already and between each stack acquisition you'd need to kill 5-7 enemies affected by heat status without gaining anything. I've tested it like this: 1 kill with heat status (=2 stacks since I have 2 shards) -> 20 kills without heat status -> 1 kill with heat status but no stacks, still had to kill 4-5 enemies affected by heat status before I could get the next stack.
And sometimes with 2 shards you get just a single stack for some reason, wow.
DE, can you at least tell us how they are supposed to work, so that we can at least be aware whether it's worth waiting for a fix or not? Is that cooldown thing between the stacks intentional or not? If intentional, then why does the game tell you about this exactly nowhere?