Actually I've tried version 1.2.3 (downgraded both libfm and pcmanfm) and behavior is the same. It seems something else is wrong, will test further this issue.
- Bug#851855: pcmanfm: F4 or "open current folder in t... olsen
- Bug#851855: pcmanfm: F4 or "open current folder... Andriy Grytsenko