Good news everyone! I was bumping into this same issue that I could replicate with an external USB drive and a network share. While the behaviour led me to believe this was a problem with an application (Kate, XnViewMP, Chromium, Dolphin, etc) or perhaps the USB device, I later discovered this is in fact a recently introduced bug with KDE Framework.
Good news because it’s being addressed in KDE Framework 5.81, so hopefully we should inherit the upstream fixes in our next batch of Manjaro updates. 
Hopefully this will solve the issue for most users.
I could 100% reproduce this issue on KDE Framework 5.80 with Kate, XnViewMP, and Chromium browser, and it didn’t matter if the device was a USB drive or a network share. As noted by @Sixtiz above, this also affects iSCSI.
Here are the bug reports for reference:
https://bugs.kde.org/show_bug.cgi?id=434455
https://bugs.kde.org/show_bug.cgi?id=434781
https://bugs.kde.org/show_bug.cgi?id=434797
https://bugs.kde.org/show_bug.cgi?id=434801
https://bugs.kde.org/show_bug.cgi?id=435169
If the problem still persists after KDE Framework 5.81, we should share about it in here and maybe we can submit a newer bug report.