CiraFalabella - I have had similar situation recently while working with schedule updates from MSP to CA PPM. I have identified Task Calendar was creating an issue in my case. Base Calendar set in PPM excludes holidays while MSP's Task Calendar when set as '24 hours' it gives up different date to what is there in Clarity.
Scenario was like we had project cut over dates over weekend, which was correctly showing up in MSP and when saved back to PPM, we had no issues. However, when we exported the same schedule again and updated %complete for few other tasks, and did not update the dates/task calendar settings in MSP for cut over tasks and saved the schedule back to CA PPM dates were getting changed. The reason for this was we don't have an option to save Task Calendar mapping to Clarity. I have raised a support case to CA to check how these task level calendar settings can be handled.
Similar to above scenario please check if this is something to do with task calendar in your case. If you have made any progress in resolving this, please share the solution.
Thanks,
Praneeth