As I said above, this is not happening on timelines, this is happening when creating a historic event from non-timeline view.
An example of timeline that I am unable to add an event created that way to is https://www.worldanvil.com/p/timeline/edit/2fe1d7b9-3e73-470c-9914-ee1cf0a0df81 but I don't believe this is anything to do with that timeline or any of the others affected. Looking over the list of timelines available to add an event to, every timeline I would expect beginning with a number or A-G is present, and none after that. There is nothing that is consistently different about those timelines compared to ones further down the list, so the issue is definitely appears to be that the list is simply limited to only display a certain number of entries even if a world has more timelines than that.