• Construct's Armory: 1.2.5.9
• Tinkers' Construct: 2.13.0.183
• Mantle: 1.3.3.55
• Forge: 14.23.5.2847

Observed Behavior:

When you replace a plate material of an armor that has diamond modifier, it seems not to display the correct toughness value.

Expected Behavior:

The action should replace the toughness value to that of the new plate material, plus two from the diamond modifier.

Steps to Reproduce:

1. Create an arbitrary armor set with obsidian plates (toughness 3.5)
2. Add a diamond modifier to the armor item
3. Replace the armor set with sponge plates. Ideally, the toughness should be 7, but is instead 5 or sometimes even lower. Other materials exhibit more bizarre anomalies in toughness values.

I am unable to reproduce this. Going through your steps, my armor ends up with 7 toughness as expected. Here's a gif of me demonstrating my testing:

Can you reproduce your issue with only Construct's Armory and its dependencies installed?

Hmm... I could not currently reproduce this issue in a SP test world, but it has happened in a server, and I could have sworn that I had reproduced this error in a test world at some point. Perhaps the miscalculation came as a result of a conflict of mod settings.

In that case, it is unlikely to be an issue on your end, so I am going to close this.