A *secret* tip: after it kills you first time, another patch appears (now I am 100% sure I should've add kind of "notifications" appearing about new patch available), it would help :)
Viewing post in Do NOT pre-order games jam comments
*spoilers below*
Hmmm, sword should've been working in the invisible mode too, I just ensured: https://imgur.com/a/UQOfukW. Was you able to swing with it but it didn't kill the boss or you couldn't swing at all?