Indesign auto kerning to (-100) either side in 'space dash space' sequence - how do I stop it

I posted about this a few months ago but got nowhere because due to client confidentiality I couldnt upload the feature for people to open and look at. I can now do that and for expediency I've also added a screengrab to show what's happening. Indesign keeps kerning the 'space dash space' sequence to -100 as shown in the grabs. I don't want this to happen and it never used to happen but now it happens every time, with the caveat that somethimes it applies it both before and after the dash, whilst other times it's only after the dash. This is driving me nuts. Why is this happening and how do I stop it?

Any help is wildly appreciated.