Custom Boot Disk icon not displaying at start-up

I know it might seem like a trivial issue, but it's annoying me that I cannot see the custom chosen icon I have for my external boot drive that I have macOS installed on when in the boot drive screen on start-up (hold "alt" key).

e.g.

http://cdn.osxdaily.com/wp-content/uploads/2013/06/boot-mac-external-drive.jpg

I can see the custom icon for the internal HDD in my iMac, but not the external SSD connected via Thunderbolt. Yet when in macOS the custom icon can be seen from everywhere (Disk Utility, System Preferences > Startup Disk..)

What do I have to do to get that icon recognised?