Skin not visible in spectator mode GUI (since 1.13)

Risk of confusion

This report describes this bug only for 1.13(+).

See MC-125157 for the same bug in 1.12.2, whose cause appears to be unrelated to this report since it appeared later than this bug.

The bug

The player heads in the spectator mode GUI are not using the skin textures.

How to reproduce

1. Create a team

   /team add test

2. Join the team

   /team join test @s

3. Press any number to open the spectator mode GUI

4. Press '2' twice to select and open the team teleport