Conundrum

I am shooting an Onix Reload in .22. At ten meters it shoots very accurately. It has an adapter on the barrel for a moderator and I am using a Red dot sight. However, when a moderator is fitted, an HW, it shoots 15 centimeters high. Taking the moderator off and it shoots accurately again. On checking the moderator there does not appear to be any “clipping” and the same item on a rifle does not make any difference to the rifle’s accuracy.
I would be grateful for any explanation and any suggestions to alleviate the problem!
Thanks.