This has been a consistent and persistent problem at this intersection. I reported this same problem months ago and my ticket/issue was closed without the problem being corrected.

Thank you for contacting UDOT. We've recently resolved some problems with the detection here and made changes to the sequencing of the signal to try to better service this approach. The signal now uses any unused time on the eastbound through movement and gives it to the westbound left turn movement. However, this signal sometimes is forced to cut the westbound left turn short if there is a pedestrian present walking east or westbound on the south side of the intersection. Due this being a school area, this pedestrian movement is timed longer than the actual eastbound green time. The signal therefore has to service this eastbound movement longer than it is normally programmed to do; to do so, it has to borrow time from the next movement (in this case the westbound left turn). Due to traffic volumes at this intersection, we're unable to give this left turn movement more time. We're investigating other solutions (such as increasing the cycle length of the entire intersection, which presents its own potential issues) in order to resolve this.