I did tried renaming it to forge.jar but got same deletion/outcome (Error: Unable to access jarfile /serverdata/serverfiles/forge.jar).
Having version names are more convenient for me when juggling with multiple instances and renaming them its just additional unnecessary step. And in this case it helped me identify which version of jar I could copy over again without rebuilding whole server.
It's the first time I'm hearing about GAME_VERSION variable, added it now manually, but outcome was the same.
container also seems to always download server.jar. (not sure if this behavior is intended)
Screen-shot attached.