SaveEvars endpoint wrote to wrong eVar

Hi all, 

We use Adobe Analytics API (version 1.4) to help configure clients eVars to be used specifically for our tracking events on the cleints website. One of the steps in doing this is when the client provides us with an eVar to use we make a request to the SaveEvars endpoint and give it a specific name (amongst other configuration). This process has worked successfully for several customers in the past but recently we had an issue where the client provided us with eVar107 and we sent a request to the SaveEvars endpoint to configure it for our use but the client is telling us that it wrote to eVar0 instead. We're trying to understand how this could've happened since I was able to find the exact request we sent to them and the request body had "id": "107" so we would've expected it to write to eVar107. 

Now I realise that the id should maybe have been "eVar107" but it has worked previously for other clients and other eVars? I've tried to read through as much Adobe documentation as possible and I know that eVar0 is not a 'real' eVar as in the client has created it themselves but don't think that would've caused the issue here?  

Any help or insight would be greatly appreciated here or if anyone has ever seen this happen before? 

Thanks!