9.6 Atomicity of variable references Reads and writes of the following data types shall be atomic: bool, char, byte, sbyte, short, ushort,
uint, int, float, and reference types. In addition, reads and writes
of enum types with an underlying type in the previous list shall also
be atomic. Reads and writes of other types, including long, ulong,
double, and decimal, as well as user-defined types, need not be
atomic. Aside from the library functions designed for that purpose,
there is no guarantee of atomic read-modify-write, such as in the case
of increment or decrement.

9.6 Atomicity of variable references Reads and writes of the following data types shall be atomic: bool, char, byte, sbyte, short, ushort,
uint, int, float, and reference types. In addition, reads and writes
of enum types with an underlying type in the previous list shall also
be atomic. Reads and writes of other types, including long, ulong,
double, and decimal, as well as user-defined types, need not be
atomic. Aside from the library functions designed for that purpose,
there is no guarantee of atomic read-modify-write, such as in the case
of increment or decrement.

Without the release sequence rule or memory_order_release on the
fetch_sub operations, there would be nothing to require that the
stores to the queue_data were visible to the second consumer, and you
would have a data race.

very new to posting here, so apologies if it’s in the wrong place.