A lot of the time when me and my friends play online and I am hosting I'll see monsters or heroes cross the rooms diagonally through black space as they walk to the room they plan to end in.  Its annoying as it feels like it makes my defensive structures useless and we just have to gang up on them at the core!

Sometimes when we reload everything works properly but then will inevitably go back to this issue. Are there any workarounds to fix it?  @Frogsquadron any thoughts? 

I have seen this issue in the past, but usually it's my heroes taking "shortcuts" from point A to point B, and even so, it was happening very rarely.

Even so, I would suggest where possible establishing your choke points in rooms behind a crossing and not immediately on it.

EDIT: Note that this problem could also be caused by lag, so it might be worth having your friends host if you're usually the host, and vice-versa.

I thought about it being latency issues, but I'm on Fiber so I figured it probably wasn't due to me hosting poorly but we'll try.  I have all the recommend ports forwarded as well, anything else I could do on my end to reduce multiplayer lag?  

Maybe in-game lag?  I would select fullscreen but I do have a second monitor and I noticed It felt like fullscreen borderless or something, since I could move the mouse over to my second screen without alt tabbing.  Might also try installing on my SSD drive.