Thanks for the update. I still can’t reproduce it, but with enough info maybe it can be reproduce some day.
Please, add a button “Get it on F-Droid” with a PNG file from https://commons.wikimedia.org/wiki/File:Get_it_on_F-Droid.svg on https://icsdroid.bitfire.at/ and https://icsdroid.bitfire.at/download/ with the appropriate link to the page of your app on F-Droid.
You can find a link in the “ICSdroid is Open-Source Software” section of https://icsdroid.bitfire.at and its sub-pages.
Yes, but the button is is appealing too. On https://icsdroid.bitfire.at/ the “Get it on Google Play” looks a bit lonely on the large gray square. On https://icsdroid.bitfire.at/download/ it is so far “below the fold” and in a different layout that I didn’t notice it. Hence that these buttons, with the same styling, give users of all stores in one quick look all they need to know on what stores this app is available. Understandable that you want a disclaimer since it is a build you don’t do yourself. However, it is a build that is monitored very strictly and openly, so I think you can list it proudly.