Here is the issue described on Wix support.
However, the work around suggested here does not solve the problem for online stores because the customer has to scroll through two different categories.
This issue has been discussed on this forum for 3 years and there still does not seem to be any solution - this is surprising given the Classic Wix Editor has this feature neatly built in for the mobile view (see image).
Lack of this feature makes Editor X terrible for online stores!