The "bug" is still present in 1.2.6.
I have been experiencing the same issue as well (MC Windows 10 Edition, 1.2.5). Spawn rates are so low that even massive farms such as the following (on hard difficulty and in a superflat world, to make sure I only get spawns in the farm) yield at best two stacks of rotten flesh, one stack and a half of bones and a stack of gunpowder every real-time hour of gameplay.
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The farm consists of a circular ring with an inner radius of 23 blocks (inclusive of the wall) and an outer radius of 33 blocks (inclusive of the wall) and a height of two blocks. Mobs are collected through a water canal system inside of the ring and transported out (no active flushing system). I have checked the inside of the farm in operation and can cofirm that all mobs are eventually transported out (except whiches, which stay mostly still...), so after some time, the rate at which mobs fall out is the rate at which they spawn. And that rate is ridiculously low. Roughly 2 zombies, 2 spiders, 1.5 skeletons and 1 creeper every minute. There is a block in the middle where I stand and wait for the drops (I chose the inner and outer radius of the farm according to Java version spawning rules, i.e. mobs spawn and move between 24 and 32 blocks from the player). I have tried changing my vertical position with repsect to that block by +-40 blocks, but did not notice significant changes in the spawn rate
I haven't had the time to replicate this massive design on the Java edition, yet I can tell I got a similar drop rate with a basic farm consisting of 4 3x3 platforms... If we compare the efficiency in terms of mob drops per unit spawn-area, the results is that mob spawning has been reduced roughly by a factor of 40 on the Windows 10 edition (the result is simply the ratio of the spawn-eligible areas of the windows 10 farm design over the java design, given that they result in similar drop rates)! A factor 40! That basically renders mob farms utterly useless in survival mode, since the payoff is way, way too little with respect to the effort required to set up such farms.
Having said all this, it is my belief that this is not a bug, and that is an intentional feature in the Windows 10 edition to incetivize actual mob-hunting. I can understand such feature, but personally dislike it, and would like at least to be given the option to change it (e.g. ad add-on or something). More in general, I don't understand why the Windows 10 Edition should drift away from the Java version, mechanics-wise...
I seriously don't understand what is going on. For me (ever since the 1.2.0 version on Windows 10) I had the exact opposite problem, specifically for the case of mob farms. In fact, I get on average 40 times less loot from such farms in the Windows 10 edition when compared to the Java edition (no typos, I really mean 40 times less). At night too, I do not really appear to get a hell lot of mobs to spawn around me...