I equipped one of my vault dwellers with ten Stimpaks and ten Rad-Aways then sent them to explore the wasteland. As soon as they began to exit the room a Radroach attack broke out. They stopped to help fight. When it was over I had to resend them to the wasteland and all of the Stimpaks and Rad-Aways I had given them had disappeared.

I had that happen too recently, only to discover that the wasteland forager ended up with both bundles of stimpaks.