Increasing main APFS volume's size
There is nothing wrong with the Boot Camp Assistant. The problem is with the use of APFS. If the APFS configuration is not precisely correct, then the Boot Camp Assistant fails. Eventually, your macOS installation can boot and preform correctly without you knowing there is a problem with the APFS configuration. In such cases, problems first appear when trying to repartition using the Disk Utility, Boot Camp Assistant or the command line.

The stock solution is to backup using Time Machine, then reinstall everything from scratch. The result is a clean system where the Boot Camp Assistant correctly installs Windows on your Mac.

An alternate solution would be to use the Boot Camp Assistant to only download the Boot Camp Support Software (Windows Helper Software). You then would install Windows using a USB flash drive. An example of such a procedure is outlined in the accepted answer to the question "Can I install Win 10 (GPT formatted USB installer) on MacBook Pro without using BootCamp?". In your case, you have already allocated space for Windows. I would recommend modifying step 7.1 to include deleting the OSXRESERVED partition, then using the remaining free space for the Windows installation.

If you want to forgo a Windows installation, them the following commands should recover the lost space.

diskutil eraseVolume free n disk0s3
diskutil apfs resizeContainer disk0s2 0


If the above commands fail to work, then you will need to boot to Recovery Mode and enter the following commands. These commands will not alter your computer. The commands just shows the internal partition table values. Post the results to your question.

sudo fdisk /dev/disk0
sudo gpt -r show /dev/disk0