This link is already URL-escaped, but Thunder doesn’t seem to understand that and escapes the percent signs.

https://en.wikipedia.org/wiki/Ramanujan–Sato_series

Here it is again as a link on some text, which works fine.

(I edited this post from a “does this work for you?” question to a bug report.)

Update: Turns out this is caused by viewing a large image and then zooming in. At a certain zoom level the app goes into fullscreen, hiding the navigation buttons and top bar. It then never exits this mode until you quit the app.

v0.9.0-3 Pixel 7 Pro Android 16

[I edited my screenshot to add the border and the word “SCREENSHOT” because the full size preview was making it very confusing to view the post in the app. 😅]