We have a custom field in our system - "End Date"
We then installed BigGantt which added a custom field "End date"

When writing JQL, type ahead matching when you type "End" shows both fields with unique custom field ID's.

Even if we select "End Date", it still searches against "End date" - so even though the display shows unique fields based on the case, the JQL doesn't seem to recognize case in it's CF matching.

To confirm the rest of our JQL logic is correct, we renamed "End Date" to "End Date 2" and this then selects the correct data, however we cannot leave this field as End Date 2 for a pile of regression issues (filters, queries, scripts etc)