In the first image you can see the approximated damage inflicted by my Frigate to the embarked Levy (calculated strength: 5) is 19. In the second image, the approximated damage inflicted by my Frigate to the embarked Trebuchet (calculated strength: 22.4) is 5.
It seems the calculated strength of embarked melee units is always 5, so even an embarked Mech Inf can be easily sunk by Caravels. This bug is also in Vanilla, so maybe it's in the game core and it's not fixable by mods. What seems odd is the greater strength of embarked ranged units. I don't know if it's in Vanilla too or not. If not, ranged units could be tweaked to adjust their embarked defense and perhaps it also could be done with melee units.
It seems the calculated strength of embarked melee units is always 5, so even an embarked Mech Inf can be easily sunk by Caravels. This bug is also in Vanilla, so maybe it's in the game core and it's not fixable by mods. What seems odd is the greater strength of embarked ranged units. I don't know if it's in Vanilla too or not. If not, ranged units could be tweaked to adjust their embarked defense and perhaps it also could be done with melee units.