Thank you for your bug report.
The problem is in fact the same exact issue, in version 5 we replaced the stock controls with a Unicode aware set of controls, and they too suffer from the same problem.
Rather than patch the controls at a low level as we did with the stock controls I have taken a different approach this time around.
I tested this with Wine and it appears to resolve the issue for me, so it should just work for you as well.
Please try the following update, this is just the program executable so extract it over your existing flashfxp.exe
https://oss.azurewebsites.net/testr/dev-builds/ffxp5.4.0.3936.zip