Go to Options > Resource Packs . You will see "Fullbright 1.12.2 (Updated)" in the left column. Hover over it and click the arrow to move it to the right column.
No. A texture pack is client-side. It does not send packets to the server. It does not use X-Ray (which requires mods to delete block models). Fullbright simply tells your GPU to brighten the pixels. minecraft 1122 fullbright texture pack updated
Fix: You downloaded an X-Ray pack by mistake. Delete it. Genuine Fullbright keeps block textures; it only removes shadows. Go to Options > Resource Packs