Debrumaire
Thank you for your answer !
Yeah I tried not to use the <UDIM> token because I have other textures in sequence with this one and I wanted to use only this texture on UDIM 1042 but it seems that's not how it's supposed to be done.
if you have your "Emissive" textures for other UDIMs that you don't want to be used you can probably use MtlX UDIM Patch node, and compare it to 1042 using equal and multiply your texture, which will make sure that for all other UDIMs it's black
similarly you can use this technique if you want to change color of your emissive layer for eyes and differently for the top of the head, etc
in this case if you really just need 1042 you could probably keep reading just 1042 texture without <UDIM> token as you originally had it, since it'd be a waste to read other UDIMs just to multiply them by 0