In Champions > Artifact Storage

When selecting multiple artifacts to sell and selling them sometimes the selection "sticks" on the artifact slots, selecting the artifacts that now occupy those spots in storage. The game doesn't show the "Artifacts selected:" sidebar at this point and selected artifacts can be offscreen or in closed tabs. Selecting an artifact for sale adds it to this selection and pressing "Sell" sells all of them.

The "Artifacts selected:" sidebar does update to show the correct total when a new artifact is selected for sale so by paying attention to this the unwanted sale of too many  artifacts can be avoided.

The bug sometimes takes a few sales to show up and can be avoided by closing and reopening the artifact storage after each sale.