I'm not sure as I'm not involved with the SRU packaging process. It doesn't look like it's reached proposed yet.
You can still install the new mate-screensaver 1.20.2 in the meantime by using the packages built for Cosmic at 1.20.2-1 : mate-screensaver package : Ubuntu under Builds.
The downloads are:
- mate-screensaver-common_1.20.2-1_all.deb (4.4 MiB)
- mate-screensaver_1.20.2-1_amd64.deb (128.1 KiB) (64-bit only)
- mate-screensaver_1.20.2-1_i386.deb (135.9 KiB) (32-bit only)
Then install them via the terminal:
sudo dpkg -i mate-screensaver-common_*.deb mate-screensaver_*.deb
Then restart your session or computer. I just tried it in a virtual environment, and nothing broke, but I haven't got the problematic hardware to confirm the lock screen behaviour.