format includes escape quote when followed by xxx

[Valerionn]

import { format } from "date-fns"; console.log(format(d, "dd.MM.yyyy HH:mm 'UTC'xxx"));
returns something like 26.09.2018 10:07 UTC+02:00

However, when using format from date-fns-tz, i.e.
import { format as formatTZ } from "date-fns-tz"; console.log(formatTZ(d, "dd.MM.yyyy HH:mm 'UTC'xxx"));
then the second ' is included in the response, e.g. 26.09.2018 10:07 UTC'+02:00

This only seems to happen if the quotation mark is directly followed by x or z - other things like 'UTC'yyyy work without a problem.