Experiment: In Advanced Search I set Search in Forum(s) to Search All Open Forums and changed Sort Results by from (the default) in Descending Order to in Ascending Order.

Using Sort Results by: in Ascending Order, the first-displayed post (Page 1) was 11-30-2006, and the last-displayed post (Page 19) was 07-24-2013.

Using Sort Results by: in Descending Order, the first-displayed post (Page 1) was 04-23-2022, and the last-displayed post (Page 19) was 03-20-2020.

IOW, there's a gap of six years and eight months (07-24-2013 to 03-20-2020), equivalent to quite a lot of posts.

So, not "Solved", but perhaps a better perspective. IIUC, there's probably a default setting for the maximum number of posts/pages returned in a search [Gugi: hardwired? site/admin configurable?], which limits how far back/how many search results are displayed.