This article covers 

When adding metadata to a Non-Renewing subscription the subscription status changes to Active, why?

Summary 

In Chargebee, if there is a scheduled subscription change that is yet to happen and if you perform any subscription change in the meantime (before the scheduled change happens), then the subscription change (in this case- adding the metadata) will automatically remove the scheduled subscription change (cancellation in this case)- this is the current product behaviour. 

Adding the metadata to a subscription causes a subscription change event to be triggered, which cancels the scheduled subscription change automatically. This is the current expected behaviour. 

So, if you wish to add data to a subscription, instead of using the metadata (which triggers a subscription changed event), you can add a comment to the subscription: