Upgrading a world to the new format fails when it contains a junction

I have my data pack projects saved in my Dropbox, outside of my .minecraft folder. I use Windows junctions to link those data packs to my saves so that every change is immediately accessible in every world that uses that data pack. When I attempted to join my creative world today, the game gave me the option to back up my world and update, but attempting to update my world failed (claiming it was likely being accessed by another program). I made a copy of my world (which doesn’t copy the junctions), and the copy updated just fine. The cmd command used to create the junctions in my datapack folder was the following:

mklink /J "<pack title>" "S:\SmashPortal\Dropbox\datapacks\<pack title>"

This update shouldn’t be touching my datapack folder, so that shouldn’t be a roadblock.