What's that for an import script ?
Flarum relies on both post numbers and time to fetch and display the correct posts in a topic, so if your import script is giving default/non-unique values to either of these I expect your forum to not behave correctly.
Any reason for not importing the post time from the old forum ?
Also if your old forum didn't have any post numbering feature you should probably regenerate them on Flarum. It won't end well if all posts have the same number ⚠ Your import script should take care of this