This has been a problem for some time. Lightroom occasionally changes sort order.
In Library, Grid View, in the lower Tool Bar below the grid, click on the "Sort" drop down menu in the lower left corner of the grid to select the order that you are looking for. If you do not see this Tool Bar, hit the letter "t" on the keyboard.
If you see the Tool Bar but not the "Sort" Menu, click on the arrow on the right side to get this Menu: Click on "Sorting".
You can also get there by the View>Sort>
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The Import dialog (in expanded view) also has a sort order option so you see the images to import in a sorted order.
Consider this- Image files saved to camera cards or hard-drives are never in any 'sorted' order, they just fill any random space on the recording media allocated by the operating system of camera or computer. It is the software you run that arranges the files in the sort order you select.