X-plore shows file’s last modification time, but it can’t set it when copying file from elsewhere.
This would be desirable to retain file’s last modification time when copying it, and it’s not that X-plore would forget about it. It’s just not working on Android’s internal memory or memory card.
You can verify that by using other file managers, they have same problem.
One solution that X-plore uses is utilizing Root mode and set modify date from there after copying file. It works on rooted devices if X-plore is allowed to use root mode. Besides that, there is no fix for this.
Problem seems to be fixed with Android 8 and newer.