First off, you asked a closed-ended question as to whether a disc 'caused problems on a non-suspension fork'. Secondarily -- and given you and yours are a seasoned tandem team & experienced cyclists -- your curiosity regarding how a disc brake installation might present problems for a rigid fork that aren't present on suspension forks suggested you knew something about, or at least had experience with discs on suspension forks. Therefore, a simple and clear answer seemed appropriate.
Had you not been so specific in your comparitive reference to suspension forks vs. rigid forks or had you used an open-ended question I would have likely eleaborated... but that didn't seem necessary to answer the question as it was framed.
Clearly there are design considerations for rigid forks, but they're the same ones a suspension fork designer needs to consider, i.e., how to design or spec materials for the asymmetric loads, the load path of the brake energy relative to drop-out alignment and/or the need for passive axle retention, how best to incorporate the I.S. caliper mount, cable routing, etc.
Now, to be fair, there was some 'junk science' floated around regarding front-mounted discs on tandems a few years back by one of the OEMs who was also quite vocal about the suitability of certain disc brakes for tandems. Some of those writings may still be floating around and a casual reader could certainly be led to believe that there are "problems & risks" associated with using discs on road tandems and with rigid forks if they had never been exposed to the subsequent and extensive discussions -- never mind vast amounts of real-world experience -- that rebutted the purported problems and risks.
However, at least as far as current state of the art, production forks designed to support front disc installations on tandems present no special "problems" in terms of how they perform for all levels of users. In fact, Cannondale has been spec'ing OEM dual discs on their road tandems since 2003 at a time when only boutique tandem builders were just beginning to sort out how to modify chromoly forks for use with discs that addressed the design considerations. Since then, the OEM tandem builders have sourced both chromoly and composite production forks that accommodate discs for their tandems such that there are several different options for folks who believe that having a front disc brake is preferrable to a rim brake. Boutique tandem builders have simply adopted the production fork offerings, as would be expected