I also think just like as you do, this was the event that triggered cheating counter to change, but still kept me in the lovers route.
As i said strangely enough when you don't let Sam visit Liz at work (the two times) or when you don't go visit her yourself you are not kicked from Lovers (yeah this is surprising). The BJ she give count as cheating and it is normal. That's not a bug.
But i admit, it is quite a strange situation compared to similar sirtuations. For example in the first club scene if you don't intercept Matt, Liz will give him a BJ in the bathroom and you are instantly kicked out Lovers.
Another strange thing, in the last update, if you don't prevent the BJ in the club (the second scene) and make a save after that, you are still on "Lovers". But Nosy said too that certain consequences of your choices are no longuer immediate and will happen in the following updates which mean that for this BJ in particular you will be kicked later...