This is a limitation of CSS, rather than a bug in Tailwind. --spacing() is syntactical sugar for calc(var(--spacing) * <int>) and as per the specification:

The initial-value must be computationally independent.

Where computationally independent is defined as:

A property value is computationally independent if it can be converted into a computed value using only the value of the property on the element, and "global" information that cannot be changed by CSS.

Example 4

[…] On the other hand, 3em is not computationally independent, […]. Neither is a value with a var() function, because it relies on the value of a custom property.