As of today, what you observed is actually our intended behaviour.
If we inserted the content into A8 with INSERT(), we’d be putting it next to pre-existing data in cell B8 that is most likely unrelated to the new content. This will most likely result in a semantically inconsistent table.
However, we may make the behaviour more magical in the future and enforce strictly the table indicated by the header cells which are passed as a parameter - I think this was this your intended behaviour.