Explore pro 2 shows very exxagerated elevation gain.

Recently purchased Explore 2 for MTB rides. It adds like 20% or more to actual elevation gain. I compare with Komoot, Apple Watch Ultra and paper map. For example last Friday - Start 395m, end 1155m (and this data are OK on Garmin), but total ascent is reported as 955m. It is 200m too much and discrepency was growing gradually during the ride. This route was 100% uphil - no flat not down for 10km but graph in Connect shows downhill stretches. Not possible to miss it on bicycle, have vitnesses for this. All setings default except auto pause was off (as it often happens I ride below 5km/h during ascent and with autopause ascent was far too low). Secondaty problem is that grade is always shown as 0%.

Do I do something wrong? Or my unit has faulty sensors and needs replacement? Or it is known bug?