Most likely it is a puncture. I had this in two different cars with tubeless tires. The pressure will go down to 20-22 psi within a few days of filling/checking the air. The first time it happened, I drove for like a month - every week checking/filling air before realising that this was not good. I called up my tire fellow and he confirmed it was a puncture.
What happens is that there is a slow leakage of air and the shape of the tire deforms somewhat around the nail effectively sealing it and then the pressure stays at 20-22 psi. It is very unlikely for any tubeless tire to immediately deflate on getting pierced by a nail or similar object. Possibly it will go flat overnight or if by chance (it is unlikely since the tire's lower pressure enables itself to 'clamp' around the nail) the nail gets pushed out.
There is no need to even take the tire off the rim. Most tire shops have the plug/solution to fix it cold and they do it in like 10 minutes with a pair of pliers. It is a bit of a messy job though.
I now will never ever buy a car without tubeless tires. No more tire changing ever...
