just to report an issue I've encountered several times when painting include/exclude areas. Sadly, I cannot provide a precise reproducible example, I don't know where it comes from.
Basically, it seems to happen when I replace the original surface with (in my case) an updated one. (For example, I need to set up a biome on a placeholder surface before having the final terrain).
I add the new surface and remove the former one from the Surfaces list. Then it appears that the painting tool (Include with Surface Area off or Exclude with SA on) doesn't work, the circle turns red but no area is painted. I checked that the faces are not flipped, I perform a Reset Xform in case of bad scaling, etc. but with no results. If I add a second Surface (ie a basic plane), the paint tool doesn't work either on it.
If I create a new Forest object on my Surface, it works.
Edit : Below is a soluce I thought working, but it appears that your parameters are not transfered. Only the source objects are put in. Sorry for this. I let it in case someone has a better idea. [DOESN'T WORK]At least, if you also encounter this issue, here's a workaround I found, thanks to this topic : https://forum.itoosoft.com/forest-pro-(*)/combine-or-merge-forest-objects-question/msg13452/#msg13452
1- Create a new Forest object for your Surface.
2- In the tab Items Editor, switch from "Generate" to "Custom Edit" (click Ok in the warning dialog)
3- Same tab, Attach > Single or Multiple and choose the Forest object(s) that doesn't paint anymore.
4- switch back to "Generate"
You'll have to rename your new Forest object and of course, SAVE your scene before, in case of 3dsmax is having a bad day. [DOESN'T WORK]
Thanks for reading.