Strange. Are you online when you are checking the old email? Some emails pull content from the sender's server.
Try moving the email back into the Inbox. Mostly an experiment to see whether it's related to the email being in a folder or some other issue.
My method to handle this type of thing is to flag the emails, then move them out of the Inbox only when they are no longer relevant. That does result in a bit of clutter. I imagine it would be worse for you, since you're probably ording more often than I am.
On the rare occation that I want a permanent record of a purchase email, I print it as a PDF to an iCloud Drive folder. You can only do this if you are running iOS 10 or later.
- Choose to Print the email.
- When you see the preview, pinch out so that you see the document full screen.
- Tap the share icon (box with up arrow), and choose where you want to put the PDF.
I typically choose to drop it into a folder in iCloud for the Doccuments app. You could also save it to Notes (keeps it as a PDF you can open), copy it to DropBox, or even email it to yourself again as a PDF attachment (which shouldn't have that pesky formating issue).