How to prevent Alfine 8 from "skipping"/"slipping under load?
Alfine owner here whose hub is slipping after having changed gear. The Alfine is extremely susceptible to misalignment in the setup. There are two sets of yellow markers that need to be aligned. One can be adjusted using the gear barrel adjuster.

https://aseasyasridingabike.wordpress.com/2013/02/01/a-simple-guide-to-adjusting-a-shimano-nexus-hub-gear/

The other needs to be adjusted when the hub is set up. Sheldon Brown's extremely helpful web site indicates how you can achieve this.

https://www.sheldonbrown.com/nexus-mech.html

You will also need to tension the belt to the appropriate tension as mentioned in my comment and here:

https://www.montaguebikes.com/folding-bikes-blog/2016/03/adjusting-belt-tension-with-eccentric-bb/

Finally, the gear cable must be well greased as any stickiness will cause the gears not to drop in properly and skip/crunch etc. Unfortunately, I'm now of the opinion that the gear cable needs greased every 3-6 months, which is not the level of servicing I'd envisaged when purchasing! That said, it's still easier than replacing entire chain sets.