The auto-complete search feature was not working on a specific scenario with List type field on the form. While performing a search with a term, it was taking longer, eventually getting the message "invalid reference".
Also got following message on the top of the form,
Your search for "XXXXXXXX" took too long and was canceled. Please refine your search and try again
Transaction time: 15,159ms
Reason: maximum execution time exceeded
The following screenshot shows the autocomplete feature with message invalid reference in the list type field,
The auto-complete search query was cancelled due to the reason that it was taking more than 15sec. Thus it was displaying the message "invalid reference", even though the value we look for exists on the reference field.
This 15sec limitation was coming from the "Reference completer" Transaction quota rule.
Please review whether can you tune your reference qualifier on the list type field so that it gets executed before 15 sec. If that is not possible, as some time, the query can be very large, in that case getting the query executed in less than 15sec may not be possible. In those exceptional situations, you need to increase the "Max Duration" set up in the "Reference qualifier" Transaction quota rule.
How to increase the "Max Duration" of "Reference qualifier" Transaction quota rule?
For more details on Transaction Quota, refer the following document,