Naval Bomber Disappear after landing on carrier

I traded resources for some naval units - which included a carrier

I have not developed a carrier yet using research

The carrier was visible and reacted exactly as it normally would - nothing strange there

BUT ......

I landed 3 naval bombers on the carrier and they disappeared ?!?!?!?

What happened to them? They (the bombers) do not show up on the box that shows the units although the carrier does.