Q: Does Planetfall trigger Overwatch?
A: Landing does not count as movement for the purposes of triggering enemy overwatch fire. Disembarking triggers overwatch fire as normal.

Does this FAQ apply to AC landing as well?

Q: When units appear on the table, for example by teleporting, or spawning, or being summoned, do they trigger Overwatch?
A: No. Only completing a move or disembarking triggers Overwatch.

Also, note that “a move” in the context of Overwatch has been ruled to be a normal ground move. An aerospace move (planetfall or a landing aircraft) does not trigger overwatch, nor does a countercharge move during an assault.

it does present a slightly odd situation where a transport can land to contest an objective, shoot at some target yet apparently does not trigger OW fire. . . .

The answer here is no as an empty AC landing would never trigger OW.

The early interpretation of the rules allowed for players to fire their OW when they wanted either before disembarking troops or after.

This was later changed to disembarkation as being the final part of the move. But as the AC and disembarked units count as the same formation until the end of their activation both maybe shot at.

Only on the dismount.

For a history of this ruling, picture a time when Deathstrike Missile Launchers had the "no line of sight" note instead of the Indirect Fire special ability. Now picture a DS on Overwatch as an enemy aircraft loaded for assault comes onto the board. If the DS can fire before troops dismount, it's basically a 2+ autokill, ultimate air defense (and for ~800 points of SMs, you better believe people were firing both missiles to avoid the risk of a 1).

The result was Jervis' FAQ stating that for purposes of OW, only normal ground moves apply as a trigger, because he felt that was a clarification/correction he could make without directly contradicting the RAW.