Cache control - max-age goes crazy

Have decorated an Action method with ContentOutputCache attribute, and have set the Duration to 900 sec.

1) First load it`s all fine

Cache-Control:public, max-age=900

2) But after reloading the page, it changes to:

Cache-Control:public, max-age=63613596193

I have no idea, why it changes, for sure it`s not browser related.

Have disabled nearly everything (Episerver plugins, rewrites, adding custom headers) - and still it`s the same.

All I can think of is that is some crazy Episerver issue (v9.12), but it gets even worse, as it behaves the same when I change the ContentOutputCache to OutputCache.

Any tips?