X axis labels randomly being renamed "untitled" by numbers and I can't figure out why

This problem was posted by Keith Scotland a few years ago but was unresolved, and I'm experiencing the same issue, so I am reposting the question> Appreciate any help.

I have many 2D line charts with x & y axis data and label plotted accurately, but at a certain point for any new entries I add, the x axis label name displays as 'Untitled'? The y axis plots the new data correctly.

The x axis is a range of numerical dates (eg 01/01/01).

I have changed nothing in the chart for it to start doing this. I've searched all over and see people who had this problem when they created the chart and it was due to category axis labels on creation of the chart.

I had no problem when I created the charts, only months later has the problem started .