I’ve exported a selection as FLAC and the result is shifted. Please see the corresponding video report at https://youtu.be/ez23dyNLs8M.
I’m not able to reproduce the problem with Audacity 2.4.2 on Xubuntu 18.04.
Please repeat the steps in your video, and then import the exported FLAC file back into Audacity. Use the Time Shift Tool to line it up with the original (it may help if you label the original section first). We will then be able to compare the exported FLAC visually against the original. (When I do this the exported file is an exact copy, other than the expected tiny bit of “dither” noise.)