• Dynmap Version: core=3.1-457, plugin=3.1-457
• Server Version: paper 1.16.5 build 783
• Pastebin of Configuration.txt: https://pastebin.com/E5JQy2fM
• Steps to Replicate:

Key config:

fetchskins: true
refreshskins: true
skin-url: "http://175.27.160.37/avatar/player/%player%?size=32

Skin face file:
http://175.27.160.37/avatar/player/Iris?size=32
Dynmap MySQL_marker.php after a reboot:
http://175.27.160.37/map/standalone/MySQL_markers.php?marker=faces/32x32/Iris.png
They are not save. But Nginx logged a request from dynmap to the skin face file, which means dynmap actually got the correct face file but didnt handle it right, I guess.
You could using offline mode with name "Iris" to reproduce.

[√] I have looked at all other issues and this is not a duplicate
[√] I have been able to replicate this

Solved, using skin png not face png

If skin head cannot work, and nobody want to write a pr, just give me a way to disable it. Default Steve face is so disappointing.

I delete all rows in dynmap.Faces in database and reboot, problem remains.

Maybe I took a mistake. Skin-url need a skin file not a head png. I will test it later.