Membership 3.5b7 - Upgrading from level1 to level2 keeps both levels active


I am facing an issue with the Membership plugin. I upgraded to v3.5b7, but it doesn't change anything.

I have only Paypal Express as active gateway, setup as live.

I have 4 access levels (visitors, standard, elite, premier).

I have 3 subscribtions :
- STD: Finite (10 years), £0 so free, access to standard level
- ELI: Serial (1 month), £0.10, access to elite level
- PRE: Serial (1 month), £0.11, access to premier level

When upgrading from ELI to PRE, Paypal is doing its job properly: It really upgrades the subscribtion on Paypal, and asks the member to give his approval for a recurring payment of £0.11 instead of £0.10 previously.

But Membership doesn't consider this as an upgrade. The member keeps both ELI and PRE as active, so he's member of both Elite and Premier access levels.

The issue is the same when upgrading from STD (free) to any paid subscribtion. Both levels are still active (standard, and the other).

In settings,
- Stranger access level -> Visitors
- Default subscribtion for register users -> None

I also checked the tables integrity, and everything's fine.

I tried to setup my STD subscribtion as infinite or serial (I read this advice on the forum), but it doesn't change anything.

Any idea of what's going wrong, and how to solve it?

Thanks in advance for your help.

PS: it seems that I'm facing the same than: