regarding 4, i recently erased my data and restarted. please try saving, using the same item multiple times, and then loading the save. this should reproduce the bug.
I found the cause of the issue. It was caused by the [Auto Save] system. When an item was used and its quantity changed, the game would automatically save the record. However, other data was not saved at the same time, which led to this problem. This issue has now been fixed. Thank you for your feedback.