Sandbox Upgrade/Downgrade subscription

Did you find a way to do this in sandbox? It doesn't seem to work correctly for me.

when I made an upgrade subscription in SandBox env, I got an error as below:

Error Domain=SKErrorDomain Code=0 "cannot connect to iTunes Store" UserInfo=0x7f4c530 {NSLocalizedDescription=cannot connect to iTunes Store}

but a system alert told me the upgrade finished.

i get the same error, fails but popups a success message....wth!

Hello,

Did someone manage to find a solution ?
I'm encountering the same issue in sandbox and testflight when I downgrade my subscription, so I can't test this functionnality at all. If I launch the inapp process again, I have an Apple popup saying that I'm already subscribed but I have a workflow to test on success.And there is no way the client agrees to go on the store without testing it ...

I've tried the solutions with restrictions enabled/disabled, sandbox account with an unverified email, testflight, ... but with no luck.

Thx !