Union command breakage in 0.55

I had the following piece of code to use the union command to merge taxonomies.

{{ $ptypes := $.Site.Taxonomies.pubtypes }}
{{ $articles := ($ptypes.small | union $ptypes.medium | union $ptypes.large).Pages }}

This used to work up to 0.54. Something in 0.55 seems to break this. I get the error:

execute of template failed: template: partials/articles.html:3:31: executing "partials/articles.html" at <union $ptypes.medium>: error calling union: union does not support slices or arrays of uncomparable types

This is not reflected in the changelogs? Is this a bug?