Platform/Target and Browser Versions: Ubuntu Linux 20.04 / Chrome 88.0.4324.182
What You Did
If both lazy loading and free mode are enabled, lazy loading may not work in some circumstances.
Basically if you move to not loaded yet slides with a mouse and completely stop moving before releasing, slides are not loaded.
I recorded a video showing it: https://youtu.be/zvfhp20sccA.
At first I'm opening a tab with an example built using current master branch of Swiper.
Then I'm opening a tab with the version patched with this proposed change.