Hi,
I am truly sorry for the inconvenience you have experienced. Some players also reported this issue before. At the cutscene, after using the ultimate power on Ryan, Ethan should be defeated automatically (as you can see each hit also hurt himself). He should not be left alive with 1 HP. In this case, please try getting one more level and put more point to his VIT or STR, making his HP becomes xxxy, or xxy (with the last digit y higher than 5, like 1506, or 967) before going into this cutscene.
Technically, this issue is caused by the damage formula, which takes 10% of Ethan's Max HP per hit (10 hits total), but it is rounded. Following by this xxxy HP formula I mentioned above, if Ethan's HP is 951 (with y = 1 < 5), the damage per hit is 95,1 and it will be rounded down to 95. After 10 hits, he will have 1 HP left and the cutscene is broken. But if his HP is 956 (with y = 6 >5), the damage per hit is 95.6 and it will be rounded up to 96, and he will definitely be defeated.
I can make another patch about this, but it is kinda dangerous to your save files, because i have not touched this game files for years (I have been busy developing the sequel of this game), so there is no guarantee that the new patch will not cause new issues, which might corrupt your current save files.