Weird behavior in swapping weapons (sling & offhand)

[Dan]

The attached character normally dual-wields a longsword and a dagger. He also has a sling, which is also a one-handed weapon. I have the three marked as M, O, and R respectively.

When there are at least two shoulder/belt slots open (aside from the one currently holding the non-wielded weapon), swapping weapons changes the longsword out for the sling, but keeps the dagger in the offhand. When there's only one slot free, however, I get a message saying that there's no place to put the dagger, and I drop all three weapons. This, despite the fact that it doesn't actually need the space for it.