Sounds odd, so to rule out issues with any other routing possibilities
In a new project, with just the headphones connected (checking okay on another device first) and you select track 8 e.g. ( it’s a short clear sound) - send to the delay and then tweak the delay to have pingpong and max width you get no pingpong ??
Obviously you’ve been trying the Track Pan (and you’re not auditioning through OB or anything like that) and it’s been no good ? Strange indeed, sorry i see you tried multiple headphones, but i’m suggesting to confirm it’s only headphones connected, no midi, no usb, nothing . Might be worth doing a Factory Reset from the startup menu before contacting support, if it persists that is