This is probably a guess. I suspect you created your tables with the table wizard, very improperly named “table styles”, i.e. you selected Default Style for your table layout.
If I create a table in your document with None table style, all my manual formatting is kept when I add a new last row.
I never use the table “styles” because they are no styles in the usual LO semantics. They rather are a set of macros to create direct formatting in every cell. When you add new rows or columns, the macros are rerun.
Because I want to control exactly how my tables and cells are formatted, I always create them with None style (i.e. no formatting macro is launched) and then format my cells with styles.
When the feature is configured for “classic” styles in some future release, I’ll review my procedure and use it if it really behaves like I expect: to be able to reformat the whole table (not individual cell content formatting) simply by changing the table style.
In recent releases, you can now change the appearance by reapplying a “style” (running again the macros with a double-click on the name) but it destroys any style formatting (applied paragraph and character styles) where it should only play with (geometric) table properties. I think that the latest development in table “styles” automatically relaunch the macros when you add a row/column so that the additional row fits in the pattern.
I tested this with Box List Blue which shows an alternating row background colour. To maintain alternation, all cells are reconfigured with direct formatting.
My recommendation: don’t use table “styles”; create your tables in None type. You can then format cell content the way you like without interference with the table wizard.
EDIT
FWIW, there are at least two bugs in https://bugs.documentfoundation.org: tdf#136532 and tdf#126008.
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