Hotlink Protection possible bug

My website is serving WebP images with fallback to JPEG using picture tag, which properly works.

When I’m testing my website on Chrome DevTools emulation in the Rendering tab to disable WebP image format, all JPEG images are returning error 403 caused by Hotlink Protection.
This does not happen with WebP images.
When I disable Hotlink Protection my JPEGs are correcty served with code 200.

I wonder if this is a Hotlilnk Protection bug or it’s caused by Chrome DevTools emulation in the Rendering tab. Any idea?

I’ve just noticed that this is happening only with custom subdomain and not with my custom domain. I don’t understand why, though.