No release notes on Model 3 yet like we had on S/X. Compiled a list from what I've gathered throughout combing the forums/Reddit. Also note, that not all of these are 50.13, most were 48.30 or 50.11 or 50.12. If you're asking about 50.12 vs 50.13 then I think it may just be minor bug fixes (unless if someone notices anything different), such as small improvements in EAP.
1. Scheduled charging: for some it was there then gone, and back again, although this was in earlier releases like 50.11
2. Normalized steering wheel pressure: Apparently, you really needed to squeeze the wheel or jerk it hard to heed the autopilot warnings, now it takes a lot less effort, like S/X
3. Fixed leaning to right issue: There are several dozen people that have reported the car veering to the right, although it sounds like an alignment issue, some report it being fixed in 50.11. I also have the same with my car, just got the 50.13 update this AM (coming from 48.30), so will test on the way home from work and report back tonight.
4. Improved backup camera: There are several posts addressing this, it seems like the "washed out" appearance is fixed and less fish-eyed.
5. Improved phone key: Some have reported improved connectivity with bluetooth phone key. Personally, I have not had any issues with this in the past two weeks, am on Google Pixel and was on 48.30
