In-Design is adding "H1" paragraph style to EVERY table is inserted, so the table headers formatting is affected by that H1 style.
I guess it chooses H1 because it's the FIRST style I created (and then modified the "basic paragraph" to modify the rest of default text).
It's like it's adding "Style_ID=1"
I tried placing the character in different places, to make sure it's not the style the cursor is in which is inherited, and no, no matter where I place the cursor, when I "insert table" the cursor is automatically placed into the first cell (header cell) and the paragraph "H1" is selected (and applied)
Adding a table with NO headers does the same, so it's not me that confused style names and applied some to the wrong place.
H1 is not used and table and cells styles are not based in it.
I don't know whether it's a bug, but IT FEELS like a bug.
How do I expect it to work?
I expect that by creating a table, NO style be applied other than the table's. (Not even the basic one)
You might have set the default paragraph style for the document (which you set by selecting a paragraph style with no text frame selected) to your H1 paragraph style.
Make sure it's on "Basic paragraph" when you select the type tool..