There are a few things that could have caused this:
1. You aren't picking the superchargers as a destination so the car isn't preconditioning.
2. It was unusually cold (and in conjunction with No. 1)
3. You are going to the superchargers with a high state of charge (60% or more)
4. The superchargers you picked are urban chargers and max out at 72KW
5. There's simply something wrong with your car.
I've been able to get the max (albeit shortly) for whatever supercharger I use (150kw v2, 250kw v3, etc) but making sure I pick it as a destination and arrive with 10% or less. Only getting 50kw is definitely unusual.
Franky, I doubt it's any of the first 4 as you mentioned you've made the drive many times over 2.5 years -- so I assume you're savvy as to tesla road trips. Probably time to get the car looked at. Before I did that, I'd probably let the battery get pretty low and take it to a local supercharger (if you have one) and check your results.
FYI, urban superchargers look like this: