F3 (find next) and Shift+F3 (find previous) work in many other editors. F3 continues to work in version 7.8.9, but Shift+F3 stopped working: it just does nothing.
I don't know what was the last version I had where Shift+F3 worked for me.
The comments explain it, so here's a summary. Shift+F3 works (v7.9.5) if you last searched with Normal or Extended mode. It doesn't work if the last search was with Regular expression mode.
The reason, to quote AdrianHHH, is:
Reverse searching for a Regex has been disabled because of problems in some cases. See item 10 in the 7.8.7 Release notes. See also this forum topic. I believe that the Notepad++ forum has had a longer discussion of the topic, but I cannot find it now.