Given a bryntum grid cell, with specific dates such as 1/1/2024 populated in the cell. If you select the cell to change the date, select the month in the header between the arrows and switch for example from January to February, and then finally pick a date such as the the 10th, the date that will be newly populated in the cell is the 10th of January rather than the 10th of February. This has been reproduced in the features demo:
https://bryntum.com/products/grid/examples/features/
Attached is a demo of the issue in an salesforce context.