WordPress allows for customization of the default URL rewrite behavior (/slug/term) with a rewrite arguments array passed to the register_taxonomy() function.
By defining values for the slug or hierarchical keys in this array, the default URL structure can be altered to suit your needs. It may also be altered such that unexpected results are returned.
When creating multiple taxonomies, it is easy to copy & paste the labels and parameters arrays for one tax and overlook the necessity to update some of the specifics for subsequent uses.
WP Codex reference:
https://codex.wordpress.org/Function_Reference/register_taxonomy#Arguments
Hierarchical taxonomy explanation via Milo:
How to enable hierarchical permalinks for hierarchical taxonomies