Di2 ultegra front ring drop down when going uphill. No pressure is applied on shifters. System works just fine on flat sections. What could be a problem?
The general reason it might drop when climbing but not the rest of the time is that one of the jobs done by the front derailleur is keeping the chain from falling off by having the inner cage positioned suitably close to restrain it. Flex in the frame, crank, and/or chainring can widen this distance. There are instances where secondary chain drop prevention devices need to be used to eliminate or minimize drop no matter how you adjust the derailleur. Note that none of the above is different between Di2 and any other front derailleur.
Presuming you're indicating the problem occurs when in the small ring, any of the installation and adjustment parameters except the high limit adjustment and trim adjustment could cause or exacerbate this. Depending on whether you've got a braze-on or band type model, these include the mounting height, support bolt engagement, clamp angle if applicable, and presence and/or correct positioning of the backup plate if applicable.
As far as this problem is concerned, the only differentiating factor with it being Di2 is that all Di2 front derailleurs have support bolts and only some non-Di2 ones do. The support bolt is partially responsible for setting the angle of the derailleur cage, which if incorrect will widen the gap between the cage and chain in the area adjacent to the chain when in the small ring, which in turn can be a cause of your problem. So that and the state of the backup plate (aka frame protector in Shimano parlance) is a good thing to check first. The plates can fall off, which is a very good thing to catch sooner than later, and which could also cause this problem to start all of a sudden.
Particularly if they're a band type or a braze-on type being used with an adapter, FDs with support bolts (and also trim adjustment in post-first-generation Di2's case) are more sensitive to being adjusted in a particular sequence than those without. The mounting angle and the support bolt engagement together determine the final angle of the cage, but there's a spec for how much the support bolt should be acting upon it. The inner limit needs to be adjusted after the correct angle has been established with the correct amount of support bolt engagement, and needs to be re-adjusted if either of the other parameters are changed, or if the height is changed. If your model has it, trim adjustment also needs to be re-adjusted after any and every mounting or cage angle position change.
With practice one learns their logic and can know whether, when, and how to go in and change just one parameter in isolation on these derailleurs, but until then it's much simpler to see them as procedurally adjusted and just follow the installation and adjustment steps in the dealer manual document for your model derailleur, which is available at si.shimano.com. When the sequence in the instructions is followed there's nothing too difficult about them, but if not it's easy to get lost. If that doesn't work, and there's nothing else like worn out drivetrain parts or poor chainline going on, then add a chain catcher.