If you search here on forum for eml you will find some threads that show that K9 is not able to handle eml files directly but rely on 3rd party apps.
I just tried: yes, I can see that forwarded mails (done from K9 by myself which might be different to what see) attached as eml file. When I click on it then chrome app opens and shows me content of it. Strange, in previous versions always my preinstalled Samsung mail app was openend…
And yes, it also shows me as 0bytes - but as I said I can download and see it.