Fastpass has always created a traffic jam when rides return from being down though, this is not a new thing. I remember on multiple occasions seeing Everest's Fastpass line extend all the way to Nemo: the Musical, before FP+.
That's the problem when you have 75% of the ride's hourly capacity previously reserved to ride something that suddenly becomes unavailable. I also remember that with legacy Fastpass, WDW would continue to distribute Fastpass tickets for that attraction (although the return time would be bumped to later). What Disneyland does, and what is much smarter, is stop distributing Fastpasses altogether in the event of a ride downtime, because you never know how long the ride will be down. This means less of a bottleneck when the ride returns from the downtime.