extracted huge list of URL's from uploaders video page that had all videos in a series but no playlist (thank you for your service, first I tried using a script to getElementsByClassName and the 'addto-queue-button' and a loop to click all the watch later buttons (I previously used similar script to purge all videos from watch later list) but both seems to fail now, so thank you thank you for you service! I will trade my use for a bug report ;)
I had 130 videos but only 128 made it to the playlist. here is a list of the url's if you'd like to replicate but there's really no need as it's more than obvious what caused the problem. I have a pdf image copy of the page, but I'm sure you'll take my word. Here is a copy paste 2 csv if you like (https://pastebin.com/f3v2p9k9).
youtube url: 'https://www.youtube.com/watch?v=XXXXXXXXXXX', where XXX… = videoID
Problem arises when videoID is prefixed with underscore ('_' ) <– aww he's so cute
anyway, underscores in the videoID are fine, just so long as they don't prefix (or lead) the videoID.
I will give the links to the two videos which did not convert properly, otherwise all 128 worked great, I'll just have to manually add the videos to the PL for now ;)
P.S. get some kind of message box that non users can post comments in. I didn't wanna tweet because didn't seem best way to get the message to you, and I almost didnt't register as I hate registering somewhere I'm only going to do one post and probably never go back and read response if any. I only registered and shared because this is me giving two bucks haha, I know I'm cheap ass but at least I bring good info.