There is no malice involved, the search limits results on 300 and the default result ordering is newest first.
To find your old stuff you can use the advanced search via the search menu at the top (
https://www.championsofregnum.com/forum/search.php) and sort results for last posting date in ascending order.
However, once you get over 600 posts with that method you will also miss those posts in the middle between the 300 newest and 300 oldest.
There are a few more options, aviable, i.e. limiting to a subsection of the forums - but its indeed a bit difficult to get them all to show.
Regards