Ctrl-u cuts the line, but Ctrl-y doesn't put it back in Vi-mode

In Vi-mode, when using Ctrl+U to cut the current input it's impossible to use Ctrl+Y afterwards to get it back: it does nothing.

If I switch back to Emacs-mode and press Ctrl+Y, I get the data back. I can then switch back to Vi-mode.

I would expect Ctrl+Y to work the same way it works with readline, or for Ctrl+U to do nothing in Vi-mode (as the vi equivalent would be dd rather than Ctrl+U).