It seems Vim 8.0.685 still have this problem. Well, I was just doing a test with Vim but I actually use Neovim most of the time. I can just stick with Neovim at the moment.
call dein#recache_runtimepath()does not fix the problem?
*dein-options-on_source* on_source (List) or (String) Load the plugin before the listed plugins are loaded. Note: The plugins must be lazy loaded plugins.
on_source()event slows your startup time.
call_hookis the thing to fire
dein#load_dict()is easy way.