var pixels = new uint[renderers.width * renderers.height];
var pixels2 = MemoryMarshal.Cast<uint, ulong>(pixels);
pixels2[idx] = ...
In NET9.0 pixels2 were Span<ulong>, but in NET10.0 a different MemoryMarshal.Cast overload is used and it is ReadOnlySpan<ulong> now, so the assignment fails.