jpegs pixelated when I export from photoshop or lightroom

Hi, I'm an amateur photographer and been into editing for some years, but I'm just returning to photoshop, and I'm having some issues with lightroom and photoshop when exporting. My files are .cr2 24mp 6000x4000, I do some basic editing in lightroom or camera raw and then do some layers in photoshop, most of my images have 2 or 3 adjustment layers and at the end I add a vignette using the camera raw filter. At first I used bridge and edited in camera raw and photoshop, but when I exported from photoshop to jpeg (I resize the file on export to 3000x2000px) but every time I do it no matter what it comes a pixelated in the transitions of the vignette shadows in the darkest corners. I tried exporting as, legacy exporting, save for web, save as copy and even the save as legacymode. I tried flattening the image before exporting, and nothing, then I tried opening directly in lightroom, then to photoshop for some adjustment layers and then going back to lightroom to add there the vignette and export from lightroom to jpeg, but I still get the same results. What am I doing wrong? Thankyou all in advance.