There is a bug that some items do not stack. The bug was noticed with objects: ores, ingots, sheets, some powders, sticks.
I assume that the bug has something to do with the "temperature". All other items that do not interact in any way with the crucible, fire, etc. stack normally.
The bug is fixed only when the item is thrown away and picked up.
Mod version - [1.20.1-3.2.7]

Please turn on enableDebug and advanced tooltips (F3 + H) in the TFC client config and show the result of hovering over two stacks that apparently do not stack. Without any specific information I am suspicious of a mod conflict as we've seen GT before fuckup our heat capabilities, see #2783