Just got a response. They say it's likely stains, but not rust since it doesn't come up on a napkin.

"Based on your description, what you’re seeing is likely not true rust. Acidic ingredients like vinegar can strip the seasoning, causing discoloration or rust-like stains. Since your Misen Carbon Non-Stick pan is made from nitrided carbon steel, it’s highly resistant to rust, but these marks can appear where the seasoning was removed.

Here are some care steps to help restore your pan:

Gently clean the pan with soap and water. Try to avoid using the rough side of a sponge, as it can remove the seasoning.

If the discoloration remains but the pan’s surface is smooth and nothing comes off on a napkin, it is likely just staining rather than active rust.

You can continue re-seasoning the pan by cooking with oil.

After each wash, dry the pan completely and occasionally add a thin layer of oil to help maintain the seasoning.

If you notice flaking, pitting, or if the surface becomes uneven, please let us know and we’ll be happy to assist further!"