As you say the files on the USB drive are not hidden but you still can't see files on the USB drive on Mac. So, it looks like the USB drive's data storage way and the system errors cause your trouble.
You've pointed out that your USB drive is formatted with FAT32. Alright, both Windows and macOS support FAT32, but you may fail to use the USB if it is formatted in Windows. Because the way the two systems organize the blocks are different, even for the same format. So, you can't see files on the USB drive on your Mac.
You are suggested to reformat the drive to APFS to have full functionality on Mac. But reformatting a USB drive on Mac will erase all data on it. Since you've already moved the files to the USB, you'd better restore the files on the USB drive with a data recovery tool.
You can use iBoysoft Data Recovery for Mac to recover your lost files from the blank USB drive. I used this tool to get back my important files from an inaccessible external drive not long ago.
- Free download, install, and open the software on your Mac.
- Connect the USB drive to your Mac.
- Select the USB drive on the app's main interface and click Search for Lost Data to scan the missing data.
- Wait until the scanning process is done, preview the scanning results, and restore your files.
After getting back your data, you can format the USB drive on Mac.
- Open Disk Utility (Launchpad > Other).
- Select your USB drive from the left sidebar of the Disk Utility panel.
- Click the top Erase button.
- Set a name for the drive, choose APFS as the format, and then click Erase.