Comparing 3.2.20.09.87 vs 220.127.116.11.80 with Notepad as the external tool, automatic reload of an externally modified file works if:
1. The file was opened via the File > Open menu item (but not, as you say, a file opened originally as a worksheet).
2. The Tools > Preferences > Environment > Automatically Reload Externally Modified Files item is checked.
So this behavior is not a bug in the sense that there has been no regression in behavior from the prior production release. I did not keep a copy of the early adopter 18.104.22.168.84 release for comparison.
SQL Developer Team