Long time, first time...
<TLDR>: (resolved?) I had an unavailable disk after upgrading and then reverting an HBA ada1 aka gptid/2725ca3e-3817-11e7-8768-00400510e336. I "replaced" it via the GUI, traceback occurred, but drive rejoined the vdev raidz2-1 without being resilvered.
It feels like...