I've got a T480s that has 40GB of RAM (32GB SODIMM + 8GiB soldered). Works fine. Is the T480 different?
T480 has 2 RAM slots and S has soldered RAM and 1 slot.
OTOH unofficially T480 (not S) can work with two 32 GB SODIMMs. I did not try that.
(I wish the AMD-based thinkpads supported ECC RAM. Ryzen 7 mobile CPUs technically allow for that.)
My T480 running NetBSD: [ 1.000000] total memory = 65411 MB