I think it is because of user error. Currently on hold on the phone to ask. We use a Hi Liner with a Protector Harness which is the 'new' H harness (I think?). We also use it with a gated buckle and the anti-submarine clip.A child safety harness is difficult to fit correctly. Research recommends using a child safety harness only in situations where it is not possible to replace your lap-only seatbelt with a lap-sash seatbelt. In this case, it is recommended to use a booster seat with an anti-submarining feature.
If you must use a child safety harness, ensure that the shoulder straps are not too tight and that the lap part of the seatbelt is very low across the thighs, otherwise it may ride up into the child's stomach area. Incorrect use of a child safety harness may cause severe submarining and direct contact between the harness system and the child's neck (see video below).