Thanks for the reply. I would have landed on the previous turn, as far as I remember. I did order naval bombardment support, but it was ineffective. I'll try it again with Church's brigade by landing in St. John and then marching over to assault.
Here is another failure: https://drive.google.com/file/d/1fzjQmdxbUlOlfwvWZlW9vxaFoCJgN6gD/view?usp=sharing
I landed Church's brigade on the previous turn with assault orders - amphibious landing as you say. Church was defeated and retreated to Terre Neuve (there is the penalty at play). On this turn Church was ordered to move to Placentia and assault Plaisance. Before his arrival the French surrendered. In spite of this, the English did not gain control of the strategic town.
There must be an ownership flag not being changed on one of the game files under a peculiar set of circumstances.
EDIT: I tried it again, this time landing in St. John first and then marching overland for the assault. The English won the battle after two months, but failed to capture Plaisance as before even after a victory that eliminated all of the defending elements.