Humming sounds with the choir sounds activate when extender lines are used for melismas.

As above, I have a score using the MuseSounds choir, where I have interspersed humming and lyrics. When I create a syllable over more than one note and use extender lines to show this eg 'My___', the sound is an ahh on the first note, followed by humming sounds on the following notes. I have created a short example below with and without humming in all four voice parts to demonstrate what I mean. Am I simply not cancelling something, or is this a bug? It had worked as expected until the last fortnight, when I noticed this problem creep in. i have tried remedies such as deleting and rewriting, but to no avail, and as you can see the problem persisted with a new score (attached)
TIA for any help.