Why does my IPhone 8 Plus automatically delete emails? How to set it to stop it?

iPhones or for that matter no phone deletes anything automatically.

Take a look at your settings on Keep

Which email account are you referring to? Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

Meanwhile look at the settings on Mail Days to sync

On the iPhone there is an option called "Mail Days To Sync." / "Remove After" This option allows you to save space on your iPhone by telling the server to stop pushing emails to your phone. There are several options to select: "No Limit" -- This option will send all the emails in your inbox and folders to your iPhone

Most mails will follow this sequence

Settings > Password & Accounts > iCloud > Mail (Scroll Down) > Advanced > Remove [Select from the options]

Settings > Password & Accounts > AOL > Account > Advanced > Remove [Select from the options]

Settings > Password & Accounts > Yahoo > Account > Advanced > Remove [Select from the options]

Except for Outlook

Settings > Password & Accounts > Outlook > Mail Days to Sync [Select from the options]

Step by step:

• Open the Settings by tapping on the Settings icon
• Scroll to Passwords & Accounts and tap on it
• Tap on the name of the account that you want to change
• Tap on Account at the top of the page (where your email address is)
• For almost any type of account tap on Advanced
• Tap on Remove and change the setting
• For MS Exchange don’t tap on Account at the top, just tap Mail Days to Save

I'm having this same problem with my Comcast email (Iphone 8 plus - IOS updated)

I tried looking for the "Mail Days to Sync" you mentioned above but cannot find it.

The instructions to find Mail days to sync is included in SravaKrA’s post under the heading Most mails will follow this sequence

I look at the "setting" and don't see the "Mail days to sync" even did a search in settings. I must be missing something.