Thanks for the work around. I agree, this should be handled by Spine itself. The way to do this is to register a context handler callback, which, upon loss and restoration of the context, reconstructs all WebGL resources (vertex/index buffers, shaders, textures) using the newly restored context.
I've tried to create a repro for losing the context, but failed so far. Do you have a repro case which I could use to repro this?