API: ISRC search not working all the times

[streamtest]

Hi, I've this following issue:
Some search we do for tracks starting from ISRC do not return any result.

How to test it:

Try to get the track with id 6j76RHtw6bJCSw1LkprA1E, the track is associated with the ISRC FRZ110100478

Now run the search with q=isrc:FRZ110100478 which is supposed to return the track above but instead will return an empty set.

https://api.spotify.com/v1/search?q=isrc:FRZ110100478&type=track

The problem does not occur on every track, for example this case is working:

https://api.spotify.com/v1/search?q=isrc:BAAA8300041&type=track