Apple Support Note addressing bendy iPad Pros:

iPad Pro cellular models now feature Gigabit-class LTE, with support for more cellular bands than any other tablet. To provide optimal cellular performance, small vertical bands or “splits” in the sides of the iPad allow parts of the enclosure to function as cellular antennas.

On how those splits are made:

These bands are manufactured using a process called co-molding. In this high-temperature process, plastic is injected into precisely milled channels in the aluminum enclosure where it bonds to micro-pores in the aluminum surface. After the plastic cools, the entire enclosure is finished with a precision CNC machining operation, yielding a seamless integration of plastic and aluminum into a single, strong enclosure.

So far, so good. But:

The new straight edges and the presence of the antenna splits may make subtle deviations in flatness more visible only from certain viewing angles that are imperceptible during normal use. These small variances do not affect the strength of the enclosure or the function of the product and will not change over time through normal use.

Tucking this link away, on the off-chance I come across an iPad Pro that exceeds “subtle deviations in flatness”.