"Missing bytes" often means corrupted bytes. If you are using RS-485:

Weeks later, the plant ran without a hiccup. On quiet nights, Mara would walk past the racks and hear the steady hum again. It sounded different now — not only the comfort of routine, but the assurance of a solved puzzle. The “bytes missing” incident joined the archive: a terse folder labeled “Modbus poll bytes missing — fixed,” complete with the story of a stubborn bug, a patient engineer, and the invisible electrical noise that almost broke the rhythm.

Modbus Poll allows an "inter-frame delay" (silence between requests). If this is set too low, the slave may still be processing the previous request and respond slowly, truncating its own transmission.