Default Warranty Issue

Hello. I am quite worried right now because I sent my phone in for repair and apparently my phone is out of warranty. Was it because I was s-off? I thought s-off had nothing to do with warranty. I feel so stupid now. I didn't know you could go from s off back to s on. The issues I am having with my HTC one is a dead pixel and the build quality. It has nothing to do with the software. The phone was given to Telus Mobility which then was sent to HTC repair. Now I get a notification that says my phone is out of warranty. Please help. My phone was also returned to stock settings and no tampered sign and locked bootloader.