It's not a bug. I'm surprised that was even suggested, as they have never worked that way.
When invoked, the <<back>> and <<return>> macros create links which navigate the story history when activated by the player, they do not navigate the history directly themselves. Thus, you cannot use them within the <<button>> or <<link>> macros.
If links are unpalatable and you really want buttons to do the same. I'd suggest either styling the links created by <<back>> and <<return>> to look like buttons or using something like the following constructs, which provide similar functionality.
A <<button>> which acts somewhat like <<back>>:
A <<button>> which acts somewhat like <<return>>: (already shown by greyelf)
<<button "Return" `previous()`>><</button>>