At some point, a patch allowed dwellers in the wasteland to realize they have 100 items and automatically head back to the vault. It's not foolproof, however, and I regularly get dwellers coming back with about 60 items while claiming to be full (just now a dweller brought back 61).
All my wasteland explorers are lvl 50 with 10 in all specials. This 'bug' does not seem to relate to total strength (dwellers with 10 STR can bring back 100 items, while dwellers with 12 or 13 have been observed bringing back around 60). I have over 400 storage, so that's not it. I keep 18 dwellers in the waste at all times.
Does anyone know what's causing this? Is it a bug or working as intended? How can I mitigate it?