I think they just either didn't know what to do with Alexander or just didn't care to, but worf also basically adopted another kid and totally forgot them too.
Anyway, character flaws are fine. Admittedly Worf has a few, and this one is a doozy, but he's not some monolithically 'good' person and is a more rounded and interesting character for it.
I think it fits as a character flaw tbh. Worf has always struggled with his heritage, his parentage, his identity. He had great adoptive human parents, but always wrestled with how to be a good Klingon, and what that even means. It's unsurprising that he isn't sure how to raise a Klingon boy, when he has so many unanswered questions about his own upbringing.
The TNG-era pic of Riker threw me for a minute, wondering if they meant of little Jean-Luc in "Future Imperfect" ... then I remembered Thaddeus and Kestra. SMH.