Search records API returning 204

Search records API returning 204

I have a record in accounts module with id 1234 and custom field custom_id 5678.

when using CRM UI, I can see the account record 1234. search by custom_id 5678 works perfectly well too. all good.

when using the search API for this particular record though, the response is 204 no content.
  1. /Accounts/search?criteria=((custom_id:equals:5678))
I tried approved and converted but no luck - still 204
  1. Accounts/search?criteria=((custom_id:equals:5678))&approved=both&converted=both
when using the COQL, the response is healthy and I do see the expected record response:
  1. "select_query": "SELECT id, Account_Name FROM Accounts WHERE custom_id = '5678'"

why I am getting 204 in the regular search record API? there is a notion in the docs:

When you create or edit a record, and search for it immediately, you may face the 204 NO CONTENT error due to a delay in indexing.

I've been waiting long enough to rule out the immediate use case so there is either something wrong with indexing and search API or there is something about search API (hence desperately trying out the approved and converted flags above)

I used dummy IDs in my examples above but it does happen with several account records in our instance, other records are fine & searchable.

any ideas?