Vary the filename/path from short (one character) to max length and run the above repro, and notice the increase in bits communicated if and only if the filename/path is long, all other factors being held constant. Same for varying the data. There is no reason why Excel.exe should be interpolating this information with all the standard telemetry and connected experience stuff disabled. Even the fact that it is occurring is interesting, and doesn’t require hypotheses for its origin.
What other explanations for this network traffic have you investigated and on what basis did you reject those explanations?
Vary the filename/path from short (one character) to max length and run the above repro, and notice the increase in bits communicated if and only if the filename/path is long, all other factors being held constant. Same for varying the data. There is no reason why Excel.exe should be interpolating this information with all the standard telemetry and connected experience stuff disabled. Even the fact that it is occurring is interesting, and doesn’t require hypotheses for its origin.