It seems that when using IMAP to communicate with a local mail application, the entire online Yahoo Inbox does not transfer to the local client - meaning that after a certain date (in my case, 2019) I do not have local copies of emails. This did not happen when using POP - old emails went into local files so it was a permanent accumulation of emails (that meant I managed old emails using archive and/or folder rules that kept the local inbox file within size limitations). It is frustrating to not be able to see back more than 4 years as some of those emails become important if there is a conflict or dispute.