Hey all, I don’t think this is really a bug but this is something that I have spent multiple hours troubleshooting TWICE now because it’s fairly unexpected behavior when compared to how renaming works with files.
The TL;DR:
When renaming the parameters within a Material Parameter Collection you MUST go and update all of the places that those parameters are referenced.
The Explanation:
These locations would be within the Collection Parameter nodes in your materials, as well as any sequences you were using to modify these values. The reason this is such an elusive issue is because the sequences and materials that these are located in don’t know that the parameter doesn’t exist under that name, and there are no pointers to automatically update them.
In my case the only modification that I made was adding “Use” to the end of my parameter name, so anywhere this was written the prefix of the name was either longer than the display area or I just didn’t notice it because it was so similar.
If you go to add a new track though and compare the MPC scalar names…

The parameters will hold the invalid reference without knowing its invalid and you can play back the animations without any errors, it just won’t do anything.
I just wanted to post this warning to hopefully save someone time, and also perhaps as a warning that utilizing MPCS with sequencers in UEFN can introduce a lot of complexity and dependencies to your project. I would try to keep their use as minimal as you can, as well as plan your naming out ahead of time so you don’t have to change it and go through this process.
-Lion