Hi, there was a slight hiccup I encountered when making a resource pack using your skin making program.
When I applied the texture I made in PMCSkin3D to the strider in-game, one of the legs was missing. I found out the issue was the little white box showing where the second leg would be in the 2D view was missing. Not only that, in 3D view it flips the first leg onto the second leg's box, unlike what happens in the version of the game I was using (1.16.5 btw)
I thoroughly enjoy using your program so I hope this can be fixed!

Here are the steps to reproduce the issue:

• I was using PMCSkin3D on a Chrome browser, Windows 10 OS.
• New Skin, Select Strider Model, Check Use Default Texture (Not sure if Default texture has anything to do with it, but I did it)
• On this step I drew a pair of pants on the strider but I'm sure that's optional.
• I used the strider retexture in a resource pack on Vanilla Java Minecraft 1.16.5
• Spawned a strider on lava (Because I didn't make a texture for the cold Strider)
• Observed the lack of leg.
  

I hope this helps in fixing the issue. :-)