I am using web 8.5 with dxa 1.5 with in tridion publications there are numbered items [pages, structure groups] to create navigation items like top menu, secondary nav etc. for that we are using sitemap model.
but same sitemap model has returned correct order of listed items for top menu but for secondary nav order of items got sorted [1st all structure groups then all pages]
In the above image lst has started with a page then structure groups as per numbers present in the cms.
in the above image list has been taken for secondary nav where list is much in sorted order. starts with all structure groups and then all pages.
It has just ignored all numbering.
Note: I have tried this with DXA 1.8 with sites 9.1 and got same behavior.