Is this API for ECDSA signature verification consistent with NIST test vectors?
Public keys in this system are very sparse: for any $x$ value there are at most two possible $y$ values satisfying $y^2 = x^3 + ax + b$ where $a$ and $b$ are the curve parameters, since the equation is quadratic in $y$.  (And $x^3 + ax + b$ has a square root at all only for some values of $x$; by Hasse's theorem, the number of points on the curve can't be very far from the number of elements in the coordinate field, so only about half of the possible $x$ values work.)

The signature API you're using presumably deals only in compressed public keys, which have maybe some formatting metadata and likely a single bit determining $y$ in one byte, together with 32 bytes to determine $x$.  For example, in the ANSI X9.62 format, the first byte is 0x02 for an even $y$ value, or 0x03 for an odd $y$ value; then the remaining 32 bytes encode $x$.