Calorie burn on chest strap HR monitor higher than assault bike burn?

Your profile says you're female, but I don't know how much you weigh or how old you are. I'll say that 110 calories for 15 minutes (440 per hour) would be a decently intense workout for me, and I've been routinely active for a long time (decade+). I'm 5'5", female, mid-130s, and a 45-minute spin class (working hard) is usually upper 200s to low 300s; when I weighed 183, I figured maybe upper 300s to low 400s.

Since you're new to exercise, I'm thinking 110 for 15 minutes could be a bit high, but 50 could be low, just guessing. (Does your HR monitor know your actual resting and tested max heart rates or have a V02max/fitness test?). Cross-check against the MFP database estimate for stationary bike for another opinion. I usually go with a "when in doubt, pick lowest exercise calorie estimate" rule of thumb while losing weight, unless I have some reason to do otherwise, but that's just me.

Maybe someone with Assault air bike experience will offer an opinion.

Could it be possible that the assault air bike was only giving you the calories above your BMR while your HR monitor was giving you your entire calorie burn? That would explain such a big difference although one seems it could be a little high and the other too low.