It looks like a bug , reported here: https://github.com/lxde/pcmanfm-qt/issues/155.
Meanwhile if you must use the very same actions that you intended to use and in the same way (the ones in
/.local/share/file-manager/actions), I suggest to switch to PCManFM 1.2.4 (no conflicts with the Qt version) or Nautilus.
Otherwise, for other actions that can be created - maybe Thunar.
Creating actions in Nautilus actions configuration tool, the same actions appear exactly in PCManFM 1.2.4 (as specified here: https://wiki.lxde.org/en/PCManFM#Add_own_options_to_the_files_context_menu). I was also able to add actions manually according to the link you posted.
Installing PCManFM-qt 0.10.0 (there's no conflict between them it seems) the toplevel menu entries are displayed but not working (the Nautilus tool can create multi-level menus/submenus but they are not even seen in PCManFM-qt 0.10.0, while in PCManFM 1.2.4 they appear just like in Nautilus). In PCManFM-qt 0.10.0 none of the added menus work, not even if manually putting the files in all-user/admin