SUMMARY
When copying medium to large files (I've seen this happen with files as "small" as 30 MB) from the host filesystem to an external USB drive, then hitting "safely remove" in the "Disks & devices" section of the system tray, or in Dolphin, files just copied on the drive are corrupted on the drive.

STEPS TO REPRODUCE
1. Mount a USB drive formatted with EXT4 or exFAT
2. With Dolphin, copy large files to it from the host filesystem
3. Wait until the file transfer progress bar popping up from the system tray signals the transfer is complete.
4. Eject the USB drive, using "Safely remove", either in Dolphin or in the "Disks & Devices" section of the system tray, and wait for it to report that the USB drive can be safely removed.
5. Unplug, then replug the USB drive and mount it.
6. See the files just transferred are corrupted.

OBSERVED RESULT
Copying large files to the drive, then ejecting and mounting it again, results in those copied files being corrupted on the USB drive.

EXPECTED RESULT
Files should not be corrupted after ejecting USB drives properly.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.10.61-1-MANJARO / 5.22.5-1
(available in About System)
KDE Plasma Version: 5.22.5
KDE Frameworks Version: 5.85.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Host FS: BTRFS
USB FS: tried freshly formatted EXT4 and exFAT

Running `sync` from the terminal before ejecting the drive seems to circumvent the issue.

Corrupted files look like they are truncated. Video files will play fine for a short time from the beginning of the video. Otherwise, many files appear as 0B long files.

The reported transfer speed by the progress bar is improbably fast.

*After* copying the files with Dolphin, but *before* ejecting the drive, if I navigate with Dolphin to the USB drive I copied files to, and look at those files, they look uncorrupted (their displayed size is correct and video files opened from Dolphin play fine). However when reopening the same USB drive after ejecting it and mounting it again, the files are corrupted.