As mentioned above, the subsection "build" of the littletiles config defaults it's values to 0 (values I tested: maxAffectedBlocks, harvestLevelBlocks, maxPlaceBlocks). The values default to 0 as soon as I enter a world. But the first time a world is opened, the user-desired values are in-effect, and not the 0, even though the config says so.

Unfortunately I don't get everything you say. You can in theory ignore the "default" configuration because you have a configuration for survival and creative, which is all you need. Not sure if there is an issue now? Could you please clarify what you mean?

Thanks for the response.

I've set up an album of screenshots with descriptions of the problem:
https://imgur.com/a/rSuGiT4

Thought that may be the fastest way to explain.

You should not use the config file. Rather use the command /cmdconfig, because if minecraft is still open it will ignore changes. Also the gui is easier to use.

Yep, that did it. I wasn't aware of the "/cmdconfig" command at all.
I've added another subsection called "adventure" and set my desired values there. The placement now works like a charm