So, I spent 10 days saving up my keys (from the Blood in the Water achievement) to loot all 10 sunken chests for the Master Diver achievement. After looting 7 chests I looked to see that I only had 4 Swim-Speed infusions. I thought that was odd and looted another chest. Saw that I looted another 3 Swim-Speed infusions, but looking in my inventory I then only had 3 Swim-Speed infusions. So, it appears that looting was replacing rather than stacking these items.
I "fixed" the problem by moving the Swim-Speed infusions to my shared inventory, and manually stacking the rest.
Anyone else seen this issue? Any way I can get the lost Swim-Speed infusions back?