It seems, or I may have missed it, that no one is adding skewness correction within the firmware and is instead using the slicer settings for calibration.
Please follow the procedure below if you still have problems and are willing to implement the official skewness correction into the printer firmware.
I assume that everyone interested in this thread is comfortable changing the printer start g-code. If not, let me know, and I will provide further details.
In summary, the G-code:
M1005 [Xn] [Yn]
M500
STL link: Bambulab Skewness Correction - Google Drive (temporary link - not controlled by me).
@BambuLab, please add the file to the BambuLab repository to ensure it isn’t lost.
Source: https://www.reddit.com/r/BambuLab/comments/1adh0bn/xy_skew_calibrationfix/
More details (e.g. revert firmware correction): Skew compensation in the Bambu X1C Firmware - #15 by klomar