Hey,
I'm not really sure about the original poster, but I am in fact still occasionally encountering them.
As always, there isn't any specific trigger whatsoever, although they still do occur every now and then.

Placing the composition in the pre-defined building slots in GM, such as the one described in the original post.

Yes, I've seen it occurring multiple times so far. Especially when it is raining in the night, although not exclusively.

Hello @Gramps,
I believe this issue has already been noted, and the next update should hopefully fix it, at least in local Game Master mode.

On a side note, this seems to temporarily go away after leaving game master, then loading it again. Or rather, it seems so, as I've usually restarted the game to be sure either way. Also, I am in fact running 16GB of RAM. And, by the way, I have also experienced these rendering issues described by the author as well, although only once. And they have happened after the lagging has already started too.

Hello,
I'm glad that I'm not the only one facing this issue - Although I am also not sure about the direct cause. Just like the author has described it, the game runs great, but then, the performance decreases heavily. And, again, is usually at the 5-10 FPS range.
I also believe that the GPU utilisation is really low in my case as well, as I've tested it multiple times. Although there doesn't seem to be a specific trigger as to something occurring in-game directly.
I don't know about the author, but I'm using the NVIDIA GeForce GTX 1650 Ti (4GB GDDR6) to play Reforger, perhaps it'll be a more little helpful. Since there's a chance it's specifically related to this model.
But yes, the issue I'm facing seems pretty much identical