I'm able to reproduce the bug (with commit
97ec2c694fe568e375ec7a2b85c1acb1e4666b54 reverted) with a small program after
viewing a minecraft apitrace.

Apparently, if 2 textures are attached to a fbo (as GL_COLOR_ATTACHMENT0,
GL_COLOR_ATTACHMENT1), GL_COLOR_ATTACHMENT1 is cleared and after that
glDrawBuffers(2, {GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1}) is called, but
the shader of the following drawing command doesn't write anything in the
second buffer (GL_COLOR_ATTACHMENT1), then it will hang.

I've attached a small code which reproduce the hang.