I confirm the problem is caused by using “table styles”. You have no other choice than formatting yourself manually your table with the help of custom or user-modified built-in paragraph styles and character styles.
So-called “table styles” are probably one of the worst designed features in Writer (personal opinion; does not involve developers). Usual styles (paragraph, character, page, frame, list) define a collection of settings under user control to be applied to the “object”. There are precedence rules between the various style categories but result can be easily altered by user and these modifications are strictly followed by Writer.
On the contrary, “table styles”, which I’d rather call “table templates”, are implemented by macros which can’t be modified by user (I use the word “macro” but they are probably code within Writer). In order to stick to the template, these macros are triggered by various events, such as row or column addition, and they repaint the whole table (this is necessary for alternating background for instance). Unfortunately, their whole formatting is done with direct formatting and don’t use styles. You may decorate your table anyway you like, the macros will always replace or override your decorations.
Personally, I stay away of this feature because it caused me more problems than real help.
One way to partially overcome the situation is to store a model/template of your table in an AutoText entry. Don’t hesitate to create an oversized table (with more columns than needed and a moderate number of rows) as it is easier to remove unused parts than to add new ones. Row number is less critical because you create a correctly formatted new row by pressing Tab in the last position of the last cell.
