Blink and WebKit reject grid-row-end 1 auto.
Edge and Firefox accept grid-row-end 1 auto and serialize it as 1 auto.

Blink and WebKit reject grid-row-end values span 1 auto, span auto 1 and 1 auto span.
Edge and Firefox accept these and serialize all of them as span 1 auto.

Testing with uppercase makes it clear that span is being accepted as a keyword and auto is being accepted as a custom identifier:
http://jsfiddle.net/ericwilligers/meya2q7d/

All browsers reject the following:
auto 1
span span 1, span 1 span and 1 span span
1 span auto, auto 1 span and auto span 1

The relevant grammar for <grid-line> is

<grid-line> =
  auto |
  <custom-ident> |
  [ <integer> && <custom-ident>? ] |
  [ span && [ <integer> || <custom-ident> ] ]

Are auto or span permitted as a <custom-ident>?

The custom-ident spec may be relevant:

Note: When designing grammars with <custom-ident>, the <custom-ident> should always be "positionally unambiguous", so that it’s impossible to conflict with any keyword values in the property.

I don't see a reading of the spec where span 1 auto would be permissible but not span 1 span.