Sorry to hear about this, could you please let us know where are you viewing the images after export?
Also, does it happen with any other image or some specific images?
Which raw image format are you working on?
Do you have any presets or plugins installed?
What are your export settings for jpg, particular the sizing?
The quality only goes down upon export and lightroom is designed to always keep the highest quality internally as it never edits the original raw file and always works from that. However, if you choose your export settings right it will be impossible to see any difference. Make sure to understand every option in the export panel. Generally an sRGB jpeg with quality of around 85 and that isn't scaled down is indistinguishable from the original. Most of the time you would send a scaled down version of the image in sRGB jpeg to a client. In that case make sure you output sharpen it. That is only necessary when you scale the image down.
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Also, to evaluate image quality, especially sharpness and noise, you have to view the image (original and export) at 1:1, where one image pixel is represented by one screen pixel. Any other view will be inaccurate and misleading because the image has been scaled.