I'm currently able to customize shulkers using the Random Entities system in Optifine 1.16.3. However, these changes are not applied to shulker boxes for some reason. I have tested with unnamed and named shulker boxes. I've seen many other people post this problem in different corners of the internet, but nobody has been able to help them. (It's honestly a very difficult problem to search on Google.) Just to be clear, I am operating in Vanilla 1.16.3 with Optifine. No other resource packs are being used.
Any help would be appreciated (and passed along to the other posts I have seen on other websites.) Thanks in advance!
After searching for a while, I found out that some of Optifine's random block entities aren't working properly in 1.16.3. Maybe try and load it in 1.16.2 and see if it works there. If it does, then there isn't anything wrong with the pack and optifine just needs updating.