Ah, now I see it. AMOS does not display the propagate buttons if there are no committable strings on the stage. But to get some committable strings, we need to propagate this contribution first (kind of chicked - egg problem).
As a quick workaround, it should help to add yet another translation change (can be even unstaged after propagating). I will appreciate an MDLSITE issue for this (or even a pull request would be great ).
Thanks in advance!