You say "Calibre 12, .... It's also the default for my documents" but that may be for a single style only. Also there is no "table default font" as you title this thread.

Could you be changing 'Styles' by accident? Switching to another style may change the font and size according to whatever that style is configured with.

A table will use the style that is in force when the table is created and if you create a second table then the style can be different in each table if you have switched styles in-between time. Also switching styles at any time will affect the font in separate cells of any table.

Next time you are having this problem check that the style has not changed from the style that you want. Style names are likely to be 'Normal', 'No spacing' and others that you see on the top ribbon. The photo shows that the 'No Spacing' style is selected.

Attached Images

Styles ribbon.jpg (39.9 KB, 25 views)