altimetro sballato dopo aggiornamento 19.01

So why don't you simply post the elevation profile demonstrating the problem? Make a screenshot of the wrong elevation profile, then turn on the Elevation Corrections, and make another screenshot of the corrected elevation profile, and post both images together here (simply copy the image and paste it into the message). In that way we should be able to compare them and see any discrepancies or problems easily.

ecco i due screen diversi, uno con il doppio del dislivello dell'altro (quello da 417 ovviamente sballato), l'orologio segna continui su e giù anche se il tragitto è senza dislivello

As I already wrote - the bumps in the profile may be caused by debris, or by the sweat or water entering the sensor port hole. Also wind or fast air flow can cause changes of the pressure (together with the debris / moisture or even alone). Make sure the sensor opening is clean, and while cycling protect it with the sleeve, with a sweat-band at the side of the watch, with a piece of fabric. Some people even put a piece of tape over the orifice, with good results.

sarebbe possibile provare a rimettere firmware precedente? per provare se è quello o meno?

Yes, sure, you just have to find the right beta installation package in the forum archive - the beta packages always include the a fallback file of the preceding public release version. Just be aware that downgrading of the firmware will reset all settings on the watch, and you will have to reconfigure it from the scratch.

yes it is possible but in this way you lose all the settings you have set up to now