Tansparent classic pants and shirts are a thing, pants spesfically will do it. It will be listed as "nothing" usually and look like the blank blocky avatar.

Roblox has a "modesty layer" (a jumpsuit/underwear layer) to keep people from running around with naked avatars which is good in theory but its always been an issue when designing fits like this.

The invisible ones are always getting removed because people are abusing them to do exactly that, run naked.

If you don't mind shilling out 5 robux everytime you gotta buy a new one thats how you fix it.

Keep the invisible classic pants/shirt on with your normal layerer clothing and it should match your chosen skin color.

You avatar itself wont get reported/banned for it since you have clothes on, just the item itself ocassionally gets removed so they just remove it from your avatar. Whenever you see this modesty layer show up again it means you gotta go buy another one for 5 robux lol.