This is the issue, for sure.
Because combat completes processing long before animations, it makes sense that animations are client side and the calculation/instruction are server generated.
It isn’t as simple as holding everyone until combat ends for everyone because then everyone would have the same lag as the worst machine/connection and that’s not fun at all. I really don’t want to play at iphoneX speed when I have a screaming fast pc for a reason.
It may be as simple as tracking disconnects and more than once in a game is considered a conceding. It may be harsh to people with bad connections, but it would be more fair to the lobby.
Another option might be to make a hard cap on how much time you can buy and sell in the tavern regardless of when the next phase starts. While this wouldn’t prevent people from getting there first, it would prevent them from some of the high apm with an extra full minute shenanigans.