Losing "sort by x-values" on closing Calc

I have an x-y scatter graph chart that I have sorted on the x-values. So far so good.
I close the file but when I open it again the chart is no longer sorted like this and the tick box is unticked. The equation for the trend line also loses formatting and position. What can I do for the chart to hold the x-value sort after closing, please?
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On re-opening
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Version: 7.1.8.1 (x64) / LibreOffice Community
Build ID: e1f30c802c3269a1d052614453f260e49458c82c
CPU threads: 4; OS: Windows 10.0 Build 19044; UI render: Skia/Raster; VCL: win
Locale: en-GB (en_GB); UI: en-GB
Calc: CL

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As saved

What file format do you use to save?, can you attach a sample file?