For the past few seasons I have had the Broken Crown on my follower with no problem, the Emantes effect is great.
I will try to explain it.
With the previous seasons, you didn’t need a gem in your own helm as long as there was a gem in your followers broken crown.
This season I have a shard in my own helm and a gem in the broken crown of the follower and it does not work. For it to work, I had to exchange my shard for a gem even though I was not wearing the broken crown. I hope you understand this.
The Broken Crown gives you extra gem drops of the same colour as the non-legendary gem in your hero’s helm. As your hero doesn’t have a non-legendary gem in their helm (because they’ve got a legendary Soul Shard) there’s nothing to drop extras of.
You can, it just has no effect on the extra gem drops - that is based on the gem in your helm. If you are running a follower without the immortality token, put an amethyst in their helm and they will get extra life to help stay alive. Since you receive a % of your followers exp, put a ruby in their helm and you will receive a small boost to exp, gained. Not much but every little bit helps.