Hello,
What you have encountered is a known issue. Unfortunately, it is difficult to replicate and the issue is still under investigation.
At the moment, there are reworks and refactorizations happening to the MODBUS services to improve and expand the functionality, However, these are expected to be included in the 7.3 firmware, which does not have a release date yet.
I suggest to try and update to the RUT2_R_00.07.02.2 firmware and see it it improves anything, as some MODBUS related changes are included in the new releases.
Best regards,
Žygimantas