Reproduction steps:

• Create a Select custom field with a global context with drop down options: Apples, Oranges
• Create an issue in project A set to Apples.
• Modify the custom field context so that only Oranges apply to project A.
• Create a new custom field context for project A with new options: Kangeroos, Zebras
• If you view the original issue again, you'll see that Apples is displayed even though only Kangeroos, Zebras are the only valid values. This generates further confusion for the user when they realize that:
  • Searching by custom field for Apples doesn't produce the issue in the results.
  • Editing the issue will simply reset the custom field to blank/empty.

It would be less confusing to not show anything at all for the custom field if this was the case.