@Jaxo
Apologies if I’m jumping to conclusions, but one way I can think of this happening is if you had headphones connected directly to outputs 1, 3 or 5 on the 18T or 12T and have the output stereo linked.
If this is how you’ve connected up – note that TRS outputs 1-6 are balanced mono outputs and cannot be used to feed headphones directly.
For stereo operation you need to connect to 1&2, 3&4 or 5&6 to get left and right signals and feeding an external headphone amp or IEM system.
If using an external headphone amp which can take a balanced mono signal to feed L&R of the headphones, you should only be using the output in mono.
Thanks,
Keith.