In an effort to enforce tighter memory safety, iOS 18 introduced new memory allocation features:
The system memory allocator (malloc(3)) has switched to a new implementation for most allocation sizes. Users might experience exposure of latent memory access bugs due to changes in heap layout, differences in performance for allocation-heavy workloads, and changes in fragmentation. (127493322)
This likely includes changes in how memory allocation for zero-sized buffers is handled to guard against information disclosure and other memory safety issues.
According to the malloc documentation:
Crashes in memory allocators are almost always related to heap corruption, such as overflowing an allocated chunk...
You are not checking the return status of the AudioConverterGetProperty function. Check that maxPacketSize is initialized and not zero as a result of this call. Also check that the size is actually sufficient.