Groundless buildings

[Ters]

When playing Simutrans earlier today I suddenly noticed that some types of buildings lacked a ground. By that I mean that the previous frame was visible through where the lawn should have been, giving that strange effect when you pan around. When opening the information dialog for the building, the gray dialog texture was visible instead.
I got this bug at the end of an hour or so long session and didn't have time to look more into it then. Most of the time was spent upgrading trains and tracks, including lots of toggling of tree and house visibility. When I briefly started the game again now, everything looked fine. I had upgraded to r4971 just prior to all this.
Do any of the regular developers have any idea what might cause this? It's possible that a make clean will cure it, but I thought I should mention it in case something more serious might be happening.
Other possibly relevant information: 64-bit Windows, 32-bit Simutrans, SDL, mingw GCC 4.5.2, pak64

[Fabio]

The building might lack needs_ground=1 parameter.

[VS]

Please mention all the specific buildings that do this to you. In order to find out if it is a bug of program, the building will have to be checked first.

[Ters]

Please mention all the specific buildings that do this!

Hard to do when they have stopped doing so. I only remember one of them: the small white modern building with a dark blue roof, it's internal name seems to be RES_01_01. Atleast one other building type was also affected.

The building might lack needs_ground=1 parameter.

I would rather say that something has taken that parameter away. At runtime. Possibly at random.

[Dwachs]

should work better with r4993.

I could not reproduce the bug, but the variable that controls whether ground is shown under buildings was not correctly initialized after loading.

[The Hood]

This does happen when that parameter is not defined, but sometimes fixes after the map has been rotated (or at least it did when I had problems with this in pak128.Britain a while back). 

[Ters]

I have managed to reproduce the bug. The ground disappears when the season changes. Upgraded to r4993 and could no longer reproduce, downgraded to r4992 and it was back, upgraded once again to r4993 and the bug is gone again. So it looks like Dwachs managed to find and kill the bug blindfolded. Well done!

[Dwachs]

Thanks