I have the same issue with my POP email account right after i updated my iPad Pro to iPadOS 13 back late Sept. all emails disappeared except for the last few days, and when i get more new emails in, those disappeared. I contacted Apple Support, and in the end they were suppose to talk to engineering, but never heard anything back. I even deleted my account and re-added it.
So i just started to keep emails on the server, so i would have all the emails on my iPad Pro. However last week, i had to download the emails to Outlook because i was running out of email space (i have my own domain with email address). So the next time i checked for email, i noticed all of those past emails are now gone, even though i have my account set to NOT delete emails. My account is setup exactly on both iPhone\iPad. I have not upgraded to 13 on my phone because of this issue. On my iPhone 8, i have emails from Mid-August, however on my iPad from last Thursday night...................
...oh wait, i just checked and now it's only showing Friday morning onward. :(