I'm seeing this in the combat odds window and in action:
Wooden Spearman with final combat strength of 12 VS Melee; killed one of my merged Obsidian Swordsmen easily. He went on to slaughter my tribal guardian; I sure regretted not having any ranged or mounted units close by.
I will provide a save the next time I see this, I see it quite often as I'm experimenting with adding that tag to unit combat classes.
Encountered the bug again; here's the save: Link
I updated my civic modmod to match what was used in this save; if you are to debug it you should apply the modmod.
The Neanderthal close to my spiked clubman has 90% vs melee when he should only have 10%.