Is the Ultegra 11-speed crankset compatible with a 12-speed groupset?
Not officially. The Ultegra 11-speed chainrings are designed for the 11-speed chain; with a Shimano 12-speed drivetrain (105/Ultegra/Dura-Ace 12-speed) the correct choice is the matching 12-speed crankset. Some people reuse it, but front shifting and chain durability suffer.
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Source: Shimano. Indicative data — always check the manufacturer's official specs.