I had a similar issue with files in a portable drive I swapped between Windows and MacOS. For some unexplained reason some files had the user immutable flag set, thus, would not be deleted even using sudo rm.
For me, what worked was to run the following command in the terminal:
sudo chflags -R nouchg '/Volumes/DriveName/DirectoryPath'
I am no expert, but put simply: this clears the “user immutable flag” on all files under the DirectoryPath. The man page should give you more details.
I discovered this in an apple forum, so credit goes to the user who posted it. Hope this helps!