Thumbnail filename is truncated at last dot (--write-thumbnail)

[nicealias]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2019.11.05
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'-f', u'worstvideo', u'--write-thumbnail', u'-o', u'%(title)s AA %(title)s BB %(title)s CC', u'https://www.youtube.com/watch?v=9J1jjz2ZgyE']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.11.05
[debug] Python version 2.7.17rc1 (CPython) - Linux-5.3.0-19-generic-x86_64-with-Ubuntu-19.10-eoan
[debug] exe versions: ffmpeg 4.1.4-1build2, ffprobe 4.1.4-1build2
[debug] Proxy map: {}
[youtube] 9J1jjz2ZgyE: Downloading webpage
[youtube] 9J1jjz2ZgyE: Downloading video info webpage
[youtube] 9J1jjz2ZgyE: Downloading thumbnail ...
[youtube] 9J1jjz2ZgyE: Writing thumbnail to: iOS 13.2 - Follow Up AA iOS 13.2 - Follow Up BB iOS 13.jpg
[debug] Invoking downloader on u'https://r8---sn-cu-ajtez.googlevideo.com/videoplayback?expire=1572962075&ei=uyrBXYjuKKSHxwKkpLXgCg&ip=213.31.102.178&id=o-APjIOac15k1-ft5bE_8Y_mV_8TXDN-QzPG_Dt9X8wAgI&itag=160&aitags=133%2C134%2C135%2C136%2C137%2C160%2C242%2C243%2C244%2C247%2C248%2C271%2C278%2C313&source=youtube&requiressl=yes&mm=31%2C26&mn=sn-cu-ajtez%2Csn-5hne6ns6&ms=au%2Conr&mv=m&mvi=7&pl=25&initcwndbps=1210000&mime=video%2Fmp4&gir=yes&clen=4222171&dur=882.014&lmt=1572766821265860&mt=1572940357&fvip=2&keepalive=yes&fexp=23842630&c=WEB&txp=5532432&sparams=expire%2Cei%2Cip%2Cid%2Caitags%2Csource%2Crequiressl%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=ALgxI2wwRAIgDZ3DodvGsrsrvKGgfKS-Cyuhw-SJI3OVqRhgawxcLAoCIBhH0OdHzcWgRrUcwoC85LkTCk5nYPp0pt_cxGGNo1KU&lsparams=mm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AHylml4wRAIgI_aW5bpMoFKTYztVUUPT0xkX2kvIo1mdhwKPVSHLLUwCIA27HiiQcjT7zkEEIKOisCh5xaOeuTcSoPJUD2StW3VH&ratebypass=yes'
[download] Destination: iOS 13.2 - Follow Up AA iOS 13.2 - Follow Up BB iOS 13.2 - Follow Up CC
[download] 100% of 4.03MiB in 00:01

Description

Please run some tests before you plainly reject/invalidate reported issues... this bug is real !!!
Note that the truncated result only shows in the thumbnail filename and not in the video filename. PS: I used worstvideo for a quick turnaround, happens with bestvideo too.
Expected thumbnail filename: iOS 13.2 - Follow Up AA iOS 13.2 - Follow Up BB iOS 13.2 - Follow Up CC.jpg
Actual thumbnail filename: iOS 13.2 - Follow Up AA iOS 13.2 - Follow Up BB iOS 13.jpg

[dstftw]

Once again: you must %(ext)s in your output template.

[nicealias]

Please reopen.
The video filename is correct. The thumbnail filename is not. The output template is the same for both!