To anyone interested in this, I did get in touch with Wix support and they confirmed that this was expected behaviour for a table on a device with a touch capability. However, they did suggest a neat way to achieve my objective.
The answer was to simply define a transparent box that sat over the top of the table headings, and ensure that the box element was more forward in the overlapping element order. That way any user keyboard action would be directed to the box element (where it is ignored) rather than to the table element.
IIt works a treat. Also, I guess by controlling whether the box is collapsed or not, I could turn this feature on or off, but I haven’t tried that so cannot confirm.