I just got off the phone with Lightroom support for this problem. They tried many things to fix it but could not so they are escalating it to their more senior developers. Tried the following, nothing fixed it:
They will call me back in 48 - 72 hours do discuss...
Current work around is to do the rename after importing the files.
Refer this similar thread : Hi, I just upgraded from LR 6.1 to LR 6.4. During importation of new files LR6.4 can't rename my files with a specific date format (importation uses only the original name).