Observed behaviour

I put in my blackstone hammer and try to switch the head to obsidian on a minecraft server which doesn't work. It says I need a tier 5 hammer to do so even though it's placed directly under a forge hammer.

Expected behaviour

I expected it to work as intended. It has fuel cells and both upgrade slots filled. What am I doing wrong?

Steps to reproduce

1. Place hammer on forge workbench.
2. ?Select hammer head.
3. Try to put in obsidian block.
4. Cannot craft because it requires tier five hammer.

Crashlog

No response

Tetra version

1.16.5-3.14.1

mGui version

3.2.0

Forge version

1.16.5

Other mods

No response

The forge hammer counts as a tier 4 hammer, combustion chamber upgrades increase its hammer tier.