I encountered this about a month or so ago. My child sim that has this issue learned to ride the bike solo while the other child sims had an adult help them. I could have sworn that I didn't have this issue before, especially with child sims who have learned to ride their bikes alone (not 100% sure as I don't do that often).
Here's the notification of Sim 1 (affected sim) and Sim 2 (unaffected sim) for Learning to ride bike.
Here is Sim 1's milestones (showing her learn to ride bike still greyed out despite getting a notification).
Here is Sim 2's milestones (showing he has learn to ride bike).
The only difference is sim 1 learned to ride a bike alone while sim 2 got their father's help.
@Sid1701d9 The sim that didn't get the milestone did learn to ride a bike on her own. She could ride around on the bike, it just doesn't show in the milestone panel that she got the milestone.
