500 page not used when there is a arc.static issue

From Discord

something kinda weird happened: i called arc.static in my head.mjs, and i made a typo in the static file name. i didn't notice an error locally in the sandbox (it wasn't a file that made things look different), but when i deployed to staging and tried to access the site, i got a nasty json internal server error message. this is unexpected for 2 reasons:
1) when i dug into the logs the error was Invoke Error {"errorType":"ReferenceError","errorMessage":"Could not find asset in static.json (asset fingerprint manifest): (which helped me to figure out I had made a typo). but why would this same error not show up in the sandbox so i have a chance to fix it, if it is going to be a showstopper in staging?
2) why was my custom 500.mjs not used and instead a random json blob shown?

https://discord.com/channels/1012099764713705472/1012435743571988560/1020948626098368643