Hay thanks for looking into it on your end.
It has not seemed to be a problem for me either on the opening (first) page on the 'newest' setting nor the 'Alphabetical' setting.
When on 'newest' setting it is pages two and five that are the ones that are easy for me to notice the problem. So for you to be able to test the same thing you would have to upload at least (i guess) 24-ish models. Or it would be around your 23-25th model that the problem would appear not your first 'Oa.stl' model.
Maybe the test to do is: note the last one on every page and then add a new one and see if the pushed one ( the one on the end of every last page) is now missing .... I will try to test that on my side.
Not sure (as I am not a web developer at all) how the page source view helps. I think I tested it and it shows that the model is not on the page.
When I search for the name of the missing models directly they do show up successfully.
I haven't added the name of these models as this seems to be a public forum. Of course, I would happily supply the name of the models direct to the developer if I get in direct contact with them.
But after this next test these model names may change ....