Cannot set specific sharing for text file
Looking at https://michielbdejong.localhost:8443/public/public-append/asdf.txt I see the text contents, not the solid-panes UI for going to the padlock / Sharing settings for that resource.
Looking at https://michielbdejong.localhost:8443/public/public-append/ I can click the padlock for the 'asdf.txt' item listed in there, but it then shows what looks like the sharing settings for https://michielbdejong.localhost:8443/public/public-append/ and not those for https://michielbdejong.localhost:8443/public/public-append/asdf.txt.
Just tried to replicate using https://khoward.dev.inrupt.net, and it appears to be working. When viewing the permissions for asdf.txt, the permissions are inherited from the parents folders (in this case 'public', but have the 'Set specific sharing for this file' button if I want to set permissions specifically for the text file.

Is there something different/odd about the permissions for the 'public' or 'public-append'? What version of mashlib are you using?
Ah i think you're right, the only thing that's incorrect then in that screenshot, and that threw me off, is that 'Set specific sharing for this file' should instead read 'Stop specific sharing for this file - just use default for public-append'
Were the permissions for asdf.txt perviously altered? If I click the 'Set specific sharing for this file' button for the file, I see:
where I then see the 'Stop specific sharing for this file - just use default for public' button ('public-append' just inherits the permissions from 'public').
So do you think the Data Browser is working as expected?