I was on dust2 on west. Silly threw an incen and it was completely invisible. I asked him about it and he said he has the Christmas skin for it. I never got to test further (for obvious reasons) but that is broken if this can be repeated. Can a staff member test this and confirm?
If this is what you're talking about it has been a known bug for a long time and was never fixed. https://www.seriousgmod.com/threads/modded-ttt-invisible-fire-when-i-have-the-textures.46535/