This is a bug in the API. It draws the table correctly, but when labeling each column, it uses the startRowIndex instead of the startColumnIndex to determine the column.

For example, if you pass this table range:

{
  "startRowIndex": 8,
  "endRowIndex": 10,
  "startColumnIndex": 0,
  "endColumnIndex": 2
}

Then the table is drawn like this:

Note that the column labels start at I, i.e. column index 8, which is what was passed for startRowIndex.

Workaround

A workaround in the meantime is to only add tables on the diagonal running from the top-left to bottom-right of the sheet. In other words, always make startRowIndex and startColumnIndex the same.