Whenever I start the game, in any map (.pak128) I get these incomplete buildings... (See link)
https://1drv.ms/u/s!AoccsHPGkzmnjTcVjeWxKpGNOoAo
I can easily remove them in "Shift+P-mode" and replace them by pressing "Increase population-button".
But, it gets really disturbing when creating big and many cities.
They will be gradually finished, albeit very slowly.
As far as Simutrans is concerned, they are not unfinished buildings, but completely ordinary ones. That they look like it to human eyes is just the pak set author's attempt at creating visual diversity and probably complementing what little support Simutrans has for buildings under construction, which doesn't "last long enough" for such tall buildings. It is just visual.
(pak64 has a similarly ugly building that can be quite prevalent, except it looks like it has collapsed or burned down or something. It is described as a brickyard and is categorized as industry, but still.)