No Bombardment Treaty prevent Invasions Too?

No

Don't try anything funny in our game unamed....

I saw something similar and puzzling a few days ago (stock, 1.33); I was invading an AI minor race home system, and they'd actually successfully built up a reasonable defense. I'd taken a planet near their homeworld but due to strategic combat weirdness, my fleet had retreated to adjacent hexes. Ground combat was successful against the minimal garrison.

The next turn a trio of AI freighters from their homeworld show up chock full of troops over the contested planet, which if they had actually landed them would have retaken the planet and made it very difficult for me to re-retake it without several turns of building more troops and sending them forward. However, the troops stayed in cargo; I sent my carrier in to try and get them in space combat before they could get to the planet, which would have failed (the troop ships reached the planet before my fighters could get to them), except it seems that they didn't actually drop the troops; my fighters killed them before they could flee combat completely, and there was no ground combat at the end of the turn.

One possibility is that the AI had those freighters set up as "reinforcements for our planet under attack", with a cargo transfer order; and wasn't smart enough to switch them to "ground assault to retake our planet" on the fly when the transfer failed due to the planet being taken.