When beginning with a new document, Highlight all cells. Right click, choose Format Cells. Set cell format to Text, click okay. Highlight/Copy tab delimited text data and paste it into document. Result: all your cells just changed back to Numeric Format.
It's doing it when attempting to import about 55 thousand lines of tab delimited data. Since there isn't an Import Option that I see, I am drag-n-dropping the very large .TXT file into a cell and then Calc prompts me with the "Text Import" wizard. I set the Text Import wizard to import Text cells and delimiter settings. Calc then populates my data but my cells are now changed back to Numeric Format. So I thought I'd work around this by creating a Template having Text Format set as a default. But still Calc still changes all the cells back to Numeric Format.
Not a bug, it's how it works. On the input window, click on the top-left of the columns' header, to select LL columns, then on type column choose text.