This is because technically 1 comes before 9, naming would need to be #102 & #097 to get sorted correctly.
I do know that some file systems account for this though and I think you're right, on how it
should be sorted, there isn't really a good way to know the number of parts something will get so #3 should come before #21.
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If I could note another thing that is semi-related, I don't think it makes much sense that sorting A-Z /newest only effects the page you are looking at. For a folder with 300+ files this is practically useless, a way for the entire folder to be sorted (even if you are only looking at 24 at a time) would be a bit nicer.
In fact is there a reason in general that viewing is limited to 24 files? Stuff like bunkr accomplishes viewing 100 at a time with an option to view every file (at the expense of a longer wait time). Allowing for the A-Z sort/newest sort to work well there.
As you can see here in jdownloader2 you can actually do both:
1) view files in order from A-Z with #97 appearing before #102 and #034 appearing
before #35 yet
after #32
2) view all files in the folder at the same time, allowing for proper sorting by size/name
is filester able to do the same?