So lots of assumptions being thrown around here so poking in to provide some clarity.
First off to address the OP. Yes we have looked into this claim, and we have not been able to replicate the behavior posted in the original post. From our testing, Stealth Armor does not provide it's stealth benefit to team mates. But with a sever caveat that as currently implemented, Stealth Armor and ECM are treated as completely separate pieces of equipment with the only interaction between them being that you need a functional ECM system in order to utilize Stealth armor.
This means that currently, if you activate Stealth Armor, your ECM is still functioning as it normally does, since it is a pre-requisite to both enable and continually use the Stealth Armor. So you are still broadcasting a standard ECM bubble that will reduce detection ranges from everyone within the bubble with the identical functionality as a standard ECM system. The stealth armor benefits in these cases would only affect the 'Mech that is actively using Stealth Armor, while team mates within the ECM bubble will gain the standard ECM effects.
In the case of BAP or the UAV, if you where to get them within the effective range of the 'Mechs, those 'Mechs under the ECM bubble but not equipped with stealth armor should be countered and targetable, but the 'Mech with active Stealth armor engaged should not since one of the perks of Stealth armor is immunity from various "auto detection" systems. In that specific case, you would need a PPC or NARC hit to disable the ECM and knock the 'Mech out of Stealth, or TAG him to make him targetable despite the active stealth armor.
This behavior has been tested internally and we are currently not seeing any deviation from what has been described above. We won't dismiss that it may be a possibility if something is not working as intended but so far we have not been able to replicate that specific behavior internally. So feel free to forward any examples along and we will look into them, but please be mindful that it is currently designed to function as described above.
Regarding this:
Khobai, on 19 December 2018 - 11:30 PM, said:
ECM is supposed to cut mechs off from sharing sensor info with their team (an ability it used to have which was removed for no good reason)
It does. This functionality was never removed. When in a Low signal state, your detection range is significantly reduced, and you do not share any targeting info with any other 'Mech within your network. The only thing they would see is that their team mate would have a "low signal" icon next to their hud info.
Additionally, Active Stealth armor puts the Stealth Armor user into a perpetual self inflicted low-signal state as long as the armor is active, which means that their own sensors are greatly diminished, and they can no longer relay targeting info to team mates.