Readonly gridpack mode gives abnormal cross-section in multi-jet process

Dear Olivier,

We are testing on the "readonly" gridpack mode to enable multithreading for event generation, as introduced in [1].
It seems the LHE cross-section given by this approach is abnormal for some multi-jet process. The discrepancy gets even larger as the jet number increases.

We prepare a W+3jet gridpack to reproduce this issue. The test is under v2.7.2 with a patch applied [2] to fix a "readonly" mode bug. The gridpack is uploaded to [3] as a reference.
For the "normal" mode, we use the command "./run 200 1 1".
For the "readonly" mode, we set the madevent folder to readonly, then do "../<folder>/run 200 1 1" in another empty directory.

The LHE cross-section given by two methods are: 1190.47 +/- 16.64 (normal) and 726.65 +/- 13.15 (readonly).
By reading the cross-section in "crossx.html", we get 1190.3 ± 2.76 fb . Hence the "readonly" result is abnormal.

Would you mind looking into this issue? Thank you very much for your help.

Best regards,
Congqiao

[1] https://answers.launchpad.net/mg5amcnlo/+question/662721
[2] https://bazaar.launchpad.net/~mg5core1/mg5amcnlo/2.7.3/revision/301
[3] https://coli.web.cern.ch/coli/tmp/.200515-212529_mtbug/w3jet_gridpack_272.tar.gz