I think the Beastmaster ability (Dellenari Beastmaster) needs to be tweaked so the effect happens at the end of a round (wears off at the end of a round). For gaining control of a creature it is fine as is but when they are controlling an opposing players beast creature you can't move the beast without the control reverting back to the owning character. This means that at the least the owning character gets the sight bonus from the creature until the next turn and the beast is back adjacent.

Example:
I have a Dellenari Beastmaster adjacent to my opponents beast creature so I control both. Owner of the creature no longer gets sight bonus from their beast creature so if they don't have another card that sees the beast they loose sight of him which is fine.
Turn 1) I move the beast and immediately the owner of the beast gets control of the creature again but since it is dimmed they can't use it howver they now see everything it sees. Of course they could do something with it now like ascent, lysis, etc.
Turn 2) I move my beastmaser creature next to my opponents beast creature and they loose the sight granted by that creature, etc.

If the effect of controlling the beast creature didn't wear off till after the round then I could have moved the beast creature and my beastmaster creature without the opponent knowing. This effect is more pronounced when the beast is question has a high sight value. Effectively half the board is visible for 1 turn then goes away once the beastmaster moves to catch up.

Generally I see this guy deal with dimwitted manticores, I don't really see any issues with it though. You can gain control of a creature, any amount of time is good.

I'd like to agree with OP, the effect should last till the end of the turn I think.