Incomplete documentation
This is not a specific bug, but more of a general observation. I discovered Sioyek back in January and promptly nuked Zathura from my system and switched over due to Sioyek's obvious superiority, and especially because of @ahrm's fast response to a serious bug I found (#1290). I have been evangelizing Sioyek to anyone who would listen. In fact, the developer coincidentally found a comment of mine on a certain social media website (that shall remain unnamed) where I extolled Sioyek's virtues and thanked me for the comment.
However, Sioyek's greatest weakness is its incomplete documentation. Once I tried to get into the weeds of tweaking my config, I found it genuinely difficult. There are so many functions that are either completely undocumented or have outdated documentation. (see example).
I kindly call on the developer to fix this problem. I even volunteer to help with the documentation because I really do believe in this program.
You are right, I will update the documentation once I have some free time.