No way to "fix" this, it was an update Facebook did mainly to fix the issue where thumbnails wouldn't show up at all at times, this was the only option they kind of had (as well as to partially encourage you to use their video service instead). Unfortunately, Twitter started doing the same thing as well.
The only way to overcome this is to manually create a copy of your thumbnail, then resize it and upload it to Facebook as a
picture but then paste the video link in the status update alongside it. The thing is, this actually works better as Facebook have been renown for pushing down direct YouTube video links (some people on here a while ago have tested it and found direct Facebook vids get more views than posting a direct YouTube link) whereas this kind of tricks Facebook into thinking you're posting a picture.
E.g.:
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