This was exactly the same python tool from a python toolbox in both versions. There was nothing that wasn't Python 3 compatible in the original python tool.
If I make a new python toolbox in Pro and run the default tool without any edits, parameters, or real code to run, I get the same behavior where it gets stuck after it completes and the geoprocessing dialog perpetually says "Updating...".
I get this problem with a default script tool in a conventional toolbox and with python toolboxes. The tool messages state that it completed...ArcPro seems to not to realize it.