SUMMARY:
This issue has been classified as a defect by Strategy. A workaround is available in the current document. A solution/code fix for this issue is not available in current releases but has been scoped for a future Strategy release.
SYMPTOM:
After saving a derived metric in a Visual Insight dashboard, if the formula uses the Case function, the spaces between the attribute form and the IN word disappears and as such the formula cannot be validated correctly.

Validation failed with syntax error! (Attribute form specified with an attribute was not found among the attribute’s forms)
SUM(CASE([Income Bracket]@DESC IN ("Over 100K"), [Cost], 0))

CAUSE:
This is a known issue in Strategy Web 10.4 and above.
ACTION:
This issue has been fixed in Strategy 10.8.
WORKAROUND:
Add the space manually for the formula to validate correctly.