One important tip before using custom elements is to check in the custom element's documentation if it requires preparation. This is the case if:

1. You intend to use a @reference on that element.
2. The element says it needs it, e.g. to apply some basic show rules.

In that case, write the following at the top of your document:

#show: e.prepare(elem1, elem2, ...)

If only custom references are needed, just #show: e.prepare() is enough.