tl;dr: The Standard Bearer enchantment is not destroyed, but doesn't have any effect on any creatures.
Warning: Rules Literalism Ahead!Standard Bearer was clearly never intended to be attached to a creature not controlled by the Warlord.
If the enchantment were worded like Death Link, this could all have been avoided.
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The literal application of the rules is that the Force Master gains control of the creature but not any of the enchantments on the creature. Mind control doesn't say anything about enchantments. It just says, "You control this creature."
Standard Bearer has a targeting restriction of "Friendly Creature" but the only times I can see that targeting restrictions for enchantments are checked is at spellcasting steps 1 and 3, and when enchantments are revealed. Those times aren't relevant here.
However, the FAQ (somewhat awkwardly) says that restricted spells can not be
1. Placed in a non-compatible spellbook before the game begins
2. Controled or "used" during the course of the game by a non-compatable mage.
No definition of "used" is given, but in the examples "used" seems to mean more than "control." It includes "cast" in the Priestess with Drain Life Wand example.
Gaining +1 Melee and +1 Armor for all other friendly creatures does seem to be "using" the Standard Bearer enchantment. Therefore, the Forcemaster's creatures will never gain the bonus from Standard Bearer, per the FAQ.
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The only question remaining is
1. Is the Standard Bearer enchantment destroyed (and cantripped back to the Warlord's book), or
2. Does it sit useless on the stolen Soldier, or
3. Does it continue to grant the Warlord's soldiers the +1 bonus?
There is no rules reason the Standard Bearer enchantment would be destroyed. The reason incompatible stolen equipment is destroyed is that it says so on Steal Equipment. It's not a general rule for incompatible stolen objects. A stolen wand keeps its incompatible spell, it's just useless.
Now we come to an unexplored area of the rules:When Standard Bearer says "All other friendly creatures in the same zone gain [+1]" does it mean friendly to the
creature or friendly to the
enchantment?
The Codex entry for friendly only says, "A friendly object is one which is currently controlled by the player [or his team]." This isn't a model of clarity, but it tells us that all objects controlled by the same player are friendly.
If Standard Bearer were written
1. "All creatures
friendly to this creature, except this creature, gain [+1]" the intent would be clear.
Likewise, if it said
2. "All creatures
friendly to this enchantment, except this creature, gain [+1]" that would also be clear.
But since it just says "friendly creatures" without saying to what they are friendly, we can only interpret that to mean, "currently controlled by you, the player". It is not immediately clear to whom the rules are speaking.
There is one clue: the bolded word
other on the card. The word "other" is where we get the "except this creature" clause in Standard Bearer's meaning. But "other" what? "
Other Creatures."
This leads me to believe that the unspoken meaning of the card is, "All
other friendly creatures [friendly to this creature] in the same zone gain +1 Melee and Armor." It's difficult to parse it as "All
other friendly creatures [friendly to this enchantment]" because "this enchantment" isn't a creature, other or otherwise.
Therefore, Standard Bearer would give other creatures friendly to the stolen Solder (i.e. controlled by the Forcemaster or his team) +1 Melee and Armor, but because you can't use Warlord Only enchantments as a Forcemaster, instead
no bonus is granted.
