It depends on the flash memory and flash controller models.
For example cheap Transcend JF500 and JF790 are writing data very slow. They may close copy dialog fast, but data is still transfers from the buffer to the flash.
I can’t believe that it is kernel issue. I hope that modern cheap TLC memory is a root cause.
Currently I know only two normal USB-flashes: Sandisk Extreme Pro and ADATA UE700. They drop their caches just after closing copy dialog. Both have LEDs, LEDs go off just after closing copy dialog.