Hey guys! Very sorry I didn’t post my solution on here sooner. Basically, it turns out that planar reflections don’t work with dynamic shadows. In our instance, ‘Dynamic Shadow Distance Stationary Light’ on our sun was turned up quite high. For those who don’t know, this allows for dynamic shadows at a distance around the player. In our instance, we were relying on this to display moving tree shadows.
Unfortunately, any shadows that fell within this radius were considered ‘dynamic’ and therefore didn’t show up in the mirror. You can actually play with this distance and see the shadows disappear or reappear in the planar reflection.
To summarise, if you bake your shadows in you’ll be fine, just be careful of ‘overriding’ your baked shadows with the dynamic shadow distance. It’s quite an annoying limitation (we struggled a lot trying to get moving tree shadows AND planar reflections in the same scene). I hope they fix it soon.