I did some more experimenting and found that if the attacking unit's strength was greater than the defending dreadnought's full strength of 22 it targeted the dreadnought. I used another Dreadnought to attack and it was targeting the Steamer - as soon as I gave it a combat 1 promo it targeted the Dreadnought as it should. I'm using BUG/BULL/BAT and BETTER AI mods - maybe it's the AI mod? Very odd.