foreign senders not showing by name on lock screen

I recently moved abroad, so I got a local SIM card but I'm still messaging my friends in my home country; I added the national calling code prefix to their number and their names turn up correctly within the Messages application.

The issue is that the +XX prefix seems to be ignored when displaying the notification in the lock screen: the sender information I get is their number (formatted as a local one, too) which is obviously quite useless.

Anyone ever encountered this? I believe this to be an OS-wide thing, but it would be useful to know more about the issue.

Thanks!