Different character encoding behaviour between plugin hosts?

I’ve run into a really weird behaviour with one of our plugins:

When we register the license key, we match the customer’s name with the license key cryptographically.
We’ve noticed that when the name contains non-standard characters, the check will fail in some plugin hosts – But not in others!

For example, in Ableton Live it works great, while in Reason and Reaper it will fail.

I am wondering how the plugin host can affect this behaviour, and if there’s anything we can do in our plugin to mitigate this?

We do convert the characters to uppercase in order to make the check non-case sensitive, and I know that this can sometimes be a problem with unicode characters. But I can’t imagine why it would work differently depending on the host?