hi,
turning on the navmesh debug mode could help a lot to find the reason, e.g. to see clearly the navmesh triangles and other boundaries (in a training video about moba AI around 48:00 there are some useful settings: Blueprint MOBA Minion/Creep AI: Lane Movement & Navmesh Debug | 03 | Live Training | Unreal Engine - YouTube).
as an easy solution, you could place invisible dynamic obstacles at the place of pawns (always remove the one corresponding to the selected character).
but it a turn based game if you need tiles, you could use a tile based A* pathfinder, you can find a solution in the turn based sample game.