Of course, this is annoying. On my X-A1, this lens worked perfectly even at F/1.2.
In your place, I would first test the lens on another Fujifilm digital camera to rule out that it is due to the X-T3. Does the X-T3 confirm the focus with the green AF field when you take the photo or do you press the shutter button immediately without waiting for the autofocus confirmation? Please take the photo once with a tripod and once without the tripod. It is quite possible that the shutter triggers a vibration that is sufficient and the photo becomes blurred by blurring. Please also check that the lens is in autofocus mode.
If all this does not help and the dealer does not want to give you your money back despite the guarantee, then you have to go to the lawyer. Unfortunately, I had to do that before.