A custom metadata key containing a double underscore — for example plate__batch — would save without complaint but could never be filtered on, added as a column, or listed in a value dropdown. The key looked fine and then quietly failed to do anything useful.
These keys are now rejected as you type them, with an inline error explaining why. Keys of that shape already in your data stay fully visible on plate, well, history, and export views so you can find and remove them; they're hidden only from the filter and column pickers, which were never able to use them.
