Hello Ginee,
Yes, the system behaviour is correct and that is the SAP Standard. If you run the current depreciation and post, then you can only run and post the "Unplanned Depreciation" for previous months. System will not allow you to run the Planned depreciation in AFAB.
You can run the "Unplanned" depreciation" for this asset for February. Once you run the 6th month planned depreciation run, this unplanned depreciation will get posted by reducing the Net book value of the asset.
Please check the same in Test system / Test mode before you post it.