Been doing a little more testing and it seems that the the issue doesn't affect PCs on VLANs 10 or 30.
Could it have something to do with VLAN 1 being the same VLAN as the modem router? So I'm technically routing through pfSense, only to route back to the same network, then out to the internet?
I did think it could have something to do with the default behaviour being different within pfSense if it detects the interface as a WAN network (as it has a gateway), but if this were the case I'd see the same issue when routing from VLAN 10 or 30 also.
I'm honestly just a bit lost. Any help would be greatly appreciated.