Dan B. is my go to guy on IGH hubs. He services these routinely whereas these days I do one once in a blue moon (if that often).
So he may be able to offer a diagnosis based on the pairs of gears lost, but I'm not.
OTOH grease on then internals can cause all sorts of issues, and complicate or mask any other causes. So I pointed out one issue, which may or may not be "THE" issue, or there may be other issues, but a proper approach is to address known causes before exploring possible unknown causes.
Look up some of Dan's videos and do a degrease and oil dip, then reassemble and see what you have (or don't).
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