Not A Bug Shallow Grave and Corpse Dance cannot be cast on opponent's end step

[Uvarovitic]

I just placed a Vintage Cube league and went to cast Shallow Grave on my opponent's end step with a creature I could legally target in my graveyard (Valgavoth, Terror Eater). Neither Shallow Grave nor Corpse Dance could be cast on my opponent's end step, although they could be cast on my subsequent turn.

 

[Neo001992]

You wouldn't happen to have a screenshot would you? Sounds like either a Grand Abolisher, Kutzil, Malamet Exemplar, or a Teferi, Time Raveler may have been in play with what you've described.