Inconsistent coloring for parenthesis?

[Aster89]

(In case any reader is interested, I originally meant to report to Vim, because in my current version the --tx below is not colored appropriately, but looking into existing issues I eventually got brought to this repo, and I have verified that the latest css.vim correct that coloring.)

In the following ruleset, the parenthesis of translate and var are colored just like translate and var, whereas the parenthesis of calc are white.

.container {
  transform: translate(calc(var(--tx, 0px) + var(--i, 0)/var(--n)*-100%));
}

I see that the highlight groups for calc's parenthesis are

['cssDefinition', 'cssAttrRegion', 'cssFunction', 'cssMathGroup', 'cssMathParens']

whereas those for var's and translate's parenthesis are respectively

['cssDefinition', 'cssAttrRegion', 'cssFunction', 'cssMathGroup', 'cssFunction', 'cssFunctionName']
['cssDefinition', 'cssAttrRegion', 'cssFunction', 'cssFunctionName']

so maybe the different coloring is intended. However I find it a bit weird.