The problem is incrementing past the index integer type limit.

Consider a simple example with ring buffer size 9, and 16bit indices:

When you increment the write index from 0xffff to 0, your "masked index" jumps from 6 (0xffff % 9) to 0 (instead of 7).

There is no elegant fix that I'm aware of (using a very wide index type, like possibly a uint64, is extremely non-elegant).