Hi, Sorry i did not respond when you commented on my post previously, i do not come on here often. I had similar findings to you, my oil was very dirty with swarf for a few changes after i built it up. I just kept riding it and changing it and eventually it went back to normal. My guess is its the gears meshing back together as they have been disturbed.
I had the bike for a good while after the issue with no other problems! Gear box never felt any different from when the washer was broken nor when i fixed it, not sure what caused it!
Best of luck hope yours sorts itself out as mine did.