It's hit or miss. I know STB and TWH's that are pure gait, they gait under saddle, they gait under lunge line, they gait in the pasture of their own accord. But SOMETIMES, usually on like an uphill climb, they might break gait and fall into a trot. It does not happen often on the horses I am personally familiar with.
My MIL tried to retrain one of her ex-race horse STB pacers, pure pacer, to trot so she could jump him. He was very resistant to trotting under saddle, even with trot poles. Was not happening.
MFT's might trot, but because their gait is a 4 beat gait, their trot is something funky. Like, you can tell when they have broken their 4 beat gait and start pacing. But I was trail riding one day and the MFT mare I was riding was doing something funky. It wasn't her gait, and it wasn't a pace. It wasn't a trot either, but something similar, like it felt like a trot without any impulsion, because I couldn't post it for the life of me and it didn't sit like a normal trot.