Another source of cracking could be related to moisture in the SMD components. We do primarily small volume PCB runs where I work. In house, with parts stored here, pick-and-placed here, etc. We have had components crack as well, and I recall one of the suspicions being the source of the components. They were a supposedly reliable supplier, but components from them failed more often than those from another supplier so we stopped getting parts from them.
I am not sure if resistors are as prone to humidity effects, but if plastic parts are not humidity controlled before going into a reflow oven, they are prone to cracking. Even as a software guy, I have seen several brand new boards with cracked components on them over the last 15 years. Often the cracked components are difficult to find, since they are so small and do not fail completely (they are just off by a wide margin.)
No, resistors are not always fully encapsulated, especially if they are small, cheap, and/or designated for high speed applications, and no, the ones that are encapsulated are not protected well enough to make high or intermittent resistance failure modes particularly unlikely after being poked. I've seen both cracking and scraping in the wild. Not as many as I've seen shorted MLCCs, but that would be a tall order.
Sure, it could also have been a design failure, but every time I've found one bad resistor in a bank of identical resistors, it has always been a physical problem with that resistor. Shrug.