Very easy to switch to Qt5 and its quite a transparent process. That would certainly isolate the widget set as an issue.
Install (from Ubuntu Repo) libqt5pas-dev - it will bring in maybe 10 meg ....
Open the Project Options, second last entry on left hand menu, Additions and Overrides. First, add a new mode to your build, the little ... button right of build modes. Call it Qt5.
Then, click "Add LCL Widget type", add Qt5, look at the matrix below there, you will see how intersecting lines and columns work, set you Qt5 build mode to assert Qt5.
Done !
Now just select that mode in the main Lazarus screen, lower button bar, near LHS and Build.
Ubuntu (and Debian) have not announced any lower level of support for GTK2 but Debian have declared it "legacy" and are strongly encouraging no new packages based on GTK2, it now generates a Lintian "Error" ?
Several of use tried it out but in my case and probably others, we just had a quick poke around, it it is something that only shows up after a delay, well, different story..
Good luck.