Hi Francesco and thank you for your report.
I was able to reproduce the issue on my test server.
Looks like trial period for variable subscriptions wasn’t implemented. In previous versions, plan edit page won’t allow you to enable trial period for variable plans. Looks like this has been changed in recent versions.
We will try to implement trial period support for variable subscriptions shortly. I will let you know when this is done. I apologies for the inconveniences this might have caused.