I've had that also yes. Connection problem and then your hits don't register. Like you said it's always happened when there is a player. The thing though is the player I was against when it happened only had a normal win loss ratio so maybe it wasn't a hack or else they'd do it more often surely? 

yeah even I have a wifi router the game also gives me the slow network message 

Hello! If the issue is constant, please report it to our Support team via https://plrm.me/arena_support or email them at: [email protected]. It would greatly help us if you could include screenshots or videos showing the issue!

I've always thought it was because my phone is bad but I guess not.

We didn't receive many complaints like this yet, so it may indeed be an issue on a player's side. Nevertheless, if it's relevant, you should contact the Support team to receive more comprehensive help!