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Describe the Bug:
I attacked with Undergrowth Champion which had 2 counters on it and AI blocked with Woodland Wanderer with 2 counters on it so 4/4 at the time.AI played Tangle to prevent all combat damage however my Undergrowth Champion still lost a counter to prevent damage to it.Same happens if I cast Tangle.
I think there is a similar report but couldn't find it.
Which card did behave improperly ?
Underworth Champion?fog like cards?other?

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
Manalink 2016/01/15: Battle For Zendikar v2,duel

What exactly should be the correct behavior/interaction ?
Undergrowth Champion

Are any other cards possibly affected by this bug ?
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Probably referring to Undergrowth Champion.

This is a legal outcome - both Tangle's and Undergrowth Champion's abilities are damage replacement effects, so they both try to replace the same event. I expect the effects get applied in timestamp order.

The controller of the object being damaged is supposed to be able to choose the order of replacement effects in 6E+ rules, and in some cases in Manalink we do that with the trigger interface, but not for damage prevention. That can't really be added without breaking things left and right unless it's done for all damage prevention/replacement effects at once, which is a huge task; plus, there's prerequisites missing.

Definitely a bug, though - not only do you not get a choice which replacement effect to apply, the automatically-chosen order is the wrong one most of the time.

In my test, if Undergrowth Champion is played BEFORE Fog, it's ability will replace the damage, but in this case Fog should done it before UC can, as it has an higher timestamp, right?

That's how it currently works in Manalink. (Except that, no, just about everything should happen earliest-timestamp first, and it does.) Comp rules say that the controller of the object being affected gets to pick the order, rule 616.

Quoted in full | Open

616. Interaction of Replacement and/or Prevention Effects

616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object's controller (or its owner if it has no controller) or the affected player chooses one to apply, following the steps listed below. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).

616.1a If any of the replacement and/or prevention effects are self-replacement effects (see rule 614.15), one of them must be chosen. If not, proceed to rule 616.1b.

616.1b If any of the replacement and/or prevention effects would modify under whose control an object would enter the battlefield, one of them must be chosen. If not, proceed to rule 616.1c.

616.1c If any of the replacement and/or prevention effects would cause an object to become a copy of another object as it enters the battlefield, one of them must be chosen. If not, proceed to rule 616.1d.

616.1d Any of the applicable replacement and/or prevention effects may be chosen.

616.1e Once the chosen effect has been applied, this process is repeated (taking into account only replacement or prevention effects that would now be applicable) until there are no more left to apply.
Example: Two permanents are on the battlefield. One is an enchantment that reads "If a card would be put into a graveyard from anywhere, instead exile it," and the other is a creature that reads "If [this creature] would be put into a graveyard from the battlefield, instead shuffle it into its owner's library." If the creature is destroyed, its controller decides which replacement to apply first; the other does nothing.
Example: Essence of the Wild reads "Creatures you control enter the battlefield as a copy of Essence of the Wild." A player who controls Essence of the Wild casts Rusted Sentinel, which normally enters the battlefield tapped. As it enters the battlefield, the copy effect from Essence of the Wild is applied first. As a result, it no longer has the ability that causes it to enter the battlefield tapped. Rusted Sentinel will enter the battlefield as an untapped copy of Essence of the Wild.

616.2. A replacement or prevention effect can become applicable to an event as the result of another replacement or prevention effect that modifies the event.
Example: One effect reads "If you would gain life, draw that many cards instead," and another reads "If you would draw a card, return a card from your graveyard to your hand instead." Both effects combine (regardless of the order they came into existence): Instead of gaining 1 life, the player puts a card from his or her graveyard into his or her hand.

How we actually want to handle it is a very open question.