Hey guys I cannot reproduce this.
I looked into the problem that I saw and I think it is different from the one you have. The bug I found was that portrait videos are not shown when I use a landscape placeholder image. This is just for videos that are added through "+Video" -> "+HTML5 Video", not Video Project thumbnails.
Are you sure you are using an h264 encoded mp4 file?
Please use a program like handbrake https://handbrake.fr/ (free) to create mp4s and then try again.
If that doesn't work please send a link to your website to email@example.com along with a link to this topic.
@adam please upload your website somewhere and send me a link to firstname.lastname@example.org with a link to this topic and a link to your website.