I run MC on two computers. I do most of the tagging on my office PC, then save tags to file and import them on the living room PC. I recently ran into two problems.
MC 14.0.50, WinXP Pro on both computers.
1. I edited the names of many video items to clean them up. I saved tags to file. The revised name appeared in the sidecar file as "Name". I updated library from tags on the other PC. Most of the items for which I had changed names did indeed change. But several of them did not. I tried Update library from tags again on those that didn't, but it didn't help.
I have just discovered that I have the very same problem as you.
I'm using WinXP Home + MC 14.0.50 on two computers. One is my work PC, and the other is the HTPC. I import and tag with the work PC, then move all to the HTPC using the "Rename etc..." window. Yesterday I struggled a lot with MC because for some unknown reason it was importing MP4 and MKV video files as Audio and as so, no matter what I tried to do, it didn't update nor create their sidecar files.
Today it started again to work correctly. I believe it restarted to work after installing ffdshow without telling me anything for the nth time. But the firewall software catched the new process so I noticed it.
From the work PC I cleaned the tags of a few videos stored on the HTPC, then updated their tags from library. I checked a few xml's and it seems all went ok. The MC running on the HTPC however didn't update automatically the library, so I selected all of them and forced MC to update the library from the tags. Some videos had their title changed to the new one, and some other refuse to change.
I did a few experiments with some MKV's and it seems to me that MC imports the tags from the sidecar only when the same tag is not already embedded into the video file.
EDIT:
I don't know if this should be reported as a bug or not. Maybe we need a new command "Update Library (from sidecar)"?