Jonathan Kehayias has an useful blog post about what is causing this problem:

https://www.sqlskills.com/blogs/jonathan/why-i-hate-the-ring_buffer-target-in-extended-events/

It doesn't give any solutions but it does explain why it's happening.

tl;dr: The missing events may still exist in memory but cannot be displayed because the size of the XML output is limited to 4 MB. Looks like it's the most recent events that are left out. Presumably as new events are added and older ones removed, the "missing" events move down the list and eventually become visible.

This would be why there appears to be a two or three day delay in seeing the deadlock details.