we tested it purri (like 200 initial spawns) and you're not really right.. there was something wrong with the spawn system (random generator wasn't really random), but that's fixed already. admins just need to update their servers..
I've noticed it happens OFTEN in opentdm that you get the same spawn if you just go ready after last map. If you vote new map and back it feels 'random' again.
Sometimes when we play 2v2 and don't change map I have gotten the same spawn 6-7 maps in a row.
Edit: I'm talking about the initial first spawn of the game, not during the game.