Custom EQ no tone heard on tone 6

Hi - my Jaybird Vistas that I received today fit well and are functional but I have issues with the custom EQ. I hear the tones up to the 5th, after which on the 6th tone I hear nothing - even with max phone volume. I hear slight crackles at the midpoint on the slider. The buds have just had a firmware update before use, and I’ve updated to iOS 13.4.1. The iOS update seems to have reduced the sibilance I’ve been experiencing. Using Spotify current version on highest sound quality I find that the “Warmth” preset where the high end is below the midline is more pleasing to my ears but I notice I lose detail. The other way round with the high range above the midline I get a lot of sibilance. I’ve found I enjoy a balance between Warmth, Rory Bosio and my own custom EQ. Perhaps the missing key is the 6th tone? I’m not an audiophile at all, but I do like the custom EQ so I’d like to use it fully with the 6th tone being functional. Any advice appreciated!