We just received a brand new Mac Mini, which came with Yosemite pre-installed. Our UPS is set to shut down the computer after running on battery backup for 1 minute. I have tested this with an iMac running Mavericks (10.9.5) and the UPS behaves as expected, but on the Mac Mini running Yosemite the computer does not shut down and just continues to run off the battery.
Updated to add: I tested with the CyberPower 825VA and the Mac Mini running OS 10.10.0. Today I installed OS 10.10.1 update and tested again with the same results. I also tested with a Furman F1500-UPS in place of the CyberPower unit and there has been no change in the behavior.
Updated 11/28/2014--I have verified in Terminal that "haltafter" is on and set to 1. I also tried triggering a shutdown by "haltlevel" 100 (shutdown when battery dips below 100%) and the machine is still not shutting down. I tried booting from an external drive that has OS 10.8.5 on it, but the Mini would not let me do this.
Updated 12/01/2014--I booted the aforementioned iMac from an external Yosemite boot drive and it displays the same behavior as the Mac Mini. No progress on the issue thus far.
Updated 12/08/2014--I have now tested 3 different brands of UPS with 3 different Macs all with the same results. Have gone back & forth with Apple support about this for the last week and they haven't informed me of any progress on this issue yet. I will update when/if this is solved.
Updated 12/15/2014--After reporting this bug to Apple and several back and forth calls with them, this issue has been passed up to the engineering team that deals with Power Management. I was told I might hear something back today if there's a quick fix or workaround, otherwise this issue may not be resolved until 10.10.2 is released.
Updated 01/22/2015--After seeing Eric's reply below, I installed OS X 10.10.2 (14C106a) beta. I only tested with 1 UPS so far, a Furman F1500-UPS, but it is now functioning as expected.