I worked on WebSphere back in the day. There were a lot of pre-compiled libraries provided by other IBM teams. I too made good use of decompilers (probably jad at the time) as it was often easier than trying to track down the source.
There were lots of "We think your patch is doing XYZZY, we see where our code should be doing that. We've updated our code and the problem went away."
Fernflower was awesome. RIP Stiver, glioblastoma can be an ugly way to die.
He opted for an assisted suicide: https://flibusta.is/node/684900.
Probably only a decade away from curing it. Unfortunately, medicine can evolve slowly.