I would also like to add that I have loaded my old save before Warble of a smitten knight quest was started and, well, started it. It appears that this quest is really the source of the problem (at least in my game version). Last time geralt's glossary entry was intact I was training to shoot and strike from horseback before signing up for the tournament.
And right after the dialog with the guy who signs you up (I have exhausted all the dialog options - about Geralt being named Ravix of fourhorn and such, and have chosen Geralt's own name instead of Ravix) Geralt's glossary entry was gone.
I have continued this quest to the point where you can choose to continue to participate in the tournament or to leave it - and O'Dimm's glossay entry was there. And after i finished the quest (I have beaten the opposing team of knights and Gregoire de Gorgon then helped Guillaume to lift the curse from Vivienne) this entry was gone too.
And one last thing: I've found and downloaded someone's save files somewhere on the internet. Warble of the smitten knight was done a bit differently there: Geralt haven't done the last part of the tournament and didn't tell Guillaume about the curse. And O'Dimms glossary entry was still in place.
So, to sum things up, it is all about some sort of bug during Warble of the smitten knight quest. I can't tell if this occurs on every system in every Witcher 3 game but my version clearly has this flaw.
Yes. This quest has been...mentioned before. It's actually helpful that you were able to track down the problem occurring at that point. Would you send that into CDPR Support along with a saved game from right before you accept the "Warble of a Smitten Knight" quest?