Screwed up while "fixing just a little thing" so had to refinish the whole blade. Stripped down the original sandblasted finish, that turned out to be a tricky way of hiding geometry flaws, and went with this "two tone" kind of kasumi. Constructive criticism appreciated.
Firstly, to clear few things up: the knife was not really losing sharpness per se, but rather losing bite. I mentioned grapefruit before, but I should have specified that I meant the inside membranes, where you need bite to cut cleanly rather than just crush or squish them. Situation with tomatoes was similar.
Loaded leather stropping was bringing that bite back almost instantly. Because of that, I had also developed a habit of touching the edge up on different stones, including finishing stones, which may have fatigued the steel right at the apex and/or occasionally created a glassy edge.
These knives are used as all-purpose kitchen knives.
To address as many possible causes as I could at once, I made an experiment on Yoshikane (White #2):
* I dulled the knife on a stone first, to fully reset the edge.
* I raised the sharpening angle to above 15 dps.
* I sharpened on diamond plates (#800 and #2000).
This one was a bit hard for me mentally, because I have been a whetstone believer for a long time, but I wanted to make sure I was properly establishing the apex while also getting a bit more pronounced toothiness from the diamonds. My regular stones are generally on softer or slower-cutting side.
* The switch in sharpening medium also forced change in technique. Even though I am pretty confident in my freehand sharpening on stones, the slower and more precise movements I used on the plates may also have helped (with both apexing and deburring).
* I bought better strop and finished on clean leather (rough and smooth side).
So far, it definitely seems to hold better than before. Thank you again for all of your replies.
Since I have you here, I would also love to hear your opinions on the best finishing style/grit for a true all-purpose kitchen knife used on everything: soft products, hard products, acidic foods, hot foods, cold foods, etc.