This is not uncommon in devops and test, but happened to me more than once in embedded code and even in other places. Sometimes I spend some official or private time investigating it, sometimes I put it into a backlog and in some cases I don't even bother due to lack of time.
How much time, if any, do you spend investigating something that is already working? Will you loose sleep over it?
In a case like that you can document everything you know and hope you have more insight when the problem comes back.
Would I lose sleep over it? No. Nothing is worth that in IT. It's just a machine, and it's just a job.