Most other decals I just use cold water but Tamiya need just slightly warm water and be very gentle with them, that's just the way I deal with them but even then I have had some that came apart.
I don't know how old the decals are but they came with a 2nd hand kit in the current box.
I use tepid water, soak the decal then set on my mat.
Agreed, usually too thick, these really looked good until wet then they just cracked, I'm wondering if the adhesive expanded under the carrier film.
Have masks on the way, but am annoyed as I thought this would be finished by tonight at the latest.