I found at my own expenses what they are. What I thought was a bug in my code that was really bugging me (pun intended), was a silly limitation of the IDE's version of sprintf(): it won't take a float for an argument. Hence, the necessity of an intermediate buffer generated by dtostrf() that is then passed to sprintf() as a string.
The fact that the OP is sending the string to an OLED is irrelevant, of course: the problem is upstream.