A ramp test isn’t a good measure for LTHR. It’s probably a decent way to find your max HR if you really, truly bury yourself until you just can’t pedal anymore due to cardiovascular fatigue (not muscular fatigue). For most of us who are trained, our legs give out first.
As @redlude97 said, LTHR is usually tested over a much longer, sustained effort. A common protocol being a 30-min max sustained (and well-paced) effort, with LTHR being the average over the final 20 mins of that effort.
I’m not a CX racer, but based on my experiences as a runner, I would expect your avg HR during CX events to be somewhat higher than a sustained cycling effort due to the additional stress of mount/dismount/carry and running, all at high effort levels. E.g., my tested LTHR on a running track is about 7bpm higher than a similar test on a bike trainer, so for me, running at 170bpm and cycling at 170bpm feel a hell of a lot different!