This is a webkit misfortunate feature:

WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.

Demo: https://jsfiddle.net/re2y30hh/
Reference: https://developer.mozilla.org/en-US/docs/Web/CSS/background-size#Browser_compatibility

Vue was set to test prefixed version before the non-prefixed by 8dd598c to fix #2219.
I think it should test non-prefixed version first, with the exception for buggy properties (like filter).

Does using non-prefixed name fix this duplicate value issue?

Yep, setProperty('background-size', '100%') works and setProperty('-webkit-background-size', '100%') does not.

@simplesmiler ah ok. So we get bitten by browser vendors either way. Oh well... yeah let's check for non-prefixed version first, and treat filter as a sepecial case.

Fixed with #2668