Comment 7 for bug 1769420

Lubuntu Cosmic will be, in many ways, very different from Lubuntu Next in all sorts of ways, including some apps. Suffice it to say, Lubuntu never actually supported a released version of LXQt. Cosmic is in development. If you want to test Lubuntu and LXQt, Cosmic is the place to do it.

If your issues are more with LXQt in general, then that is a different ball of wax altogether. Taking the LXDE version of Lubuntu and installing LXQt metapackages on it will result in a different experience. Unfortunately, that's not something the Lubuntu team does much heavy testing with, but upstream (LXQt themselves) may be able to help. There shouldn't be a lot of differences there, but artwork, settings, and additional apps will be different.

Regarding where to file bugs, this is not the appropriate place to discuss it. The <email address hidden> and/or <email address hidden> might be more appropriate. However, I will say this: if there's a bug in an app, you would generally file it against that app. The three Lubuntu-specific exceptions:
 * lubuntu-artwork
 * lubuntu-default-settings
 * lubuntu-meta (all the packages included in Lubuntu— you probably won't use this).

In your example above, filing a bug against qpdfview would be totally valid. It SHOULD be the case that qpdfview behaves as well on any desktop environment.

Here's your official advice by the way:
https://phab.lubuntu.me/w/testing/