I had to contact a GM for another reason, but I used that opportunity to ask again about the Shadow Ravens. Here's the screenshot of the conversation – it's italian, but I rewrote that in english (or use google translate if you don't trust me :P).
TL;DR – GM confirms that the short duration is not intended, it's a bug and they are working on it.
Me - “I wanted to know, is it intentional that the ravens only last a couple of seconds? As far as I know it's a bug that sould be fixed, but on the forums it's said that it may be intentional or that it will not be fixed for various reasons”
GM - “I can confirm that it's not intentional.”
GM - “The bug department is investigating the problem currently.”
Me - “So we can expect that it will soon be fixed?”
GM - “Hopefully yes.”
As I suspected, this time there were no 'promises' about an imminent fix. But at least he confirmed that it's being worked on.