It seems that this problem is not exclusive to Lubuntu.
Also tested on two different distros (Puppy and Semplice) with PCManFM and libfm 1.1.0. On both distros drag and drop doesn't work on the desktop. Same issue.
By searching PCManFM GIT logs, I see changes in the drag and drop functions. Also, when there is a problem in these functions, usually affects libfm, not pcmanfm.
It seems that this problem is not exclusive to Lubuntu.
Also tested on two different distros (Puppy and Semplice) with PCManFM and libfm 1.1.0. On both distros drag and drop doesn't work on the desktop. Same issue.
By searching PCManFM GIT logs, I see changes in the drag and drop functions. Also, when there is a problem in these functions, usually affects libfm, not pcmanfm.