OK, good to know... there may be other issues at hand, its harder to be outside trying to help solve an issue (I'm not familiar with your set up, your flows, your code, etc).
Now, in theory, a lot of cross domain traffic should transfer the ECID without the need to pass it... but there are of course outliers that still use it. For instance, if I open an incognito window, load DomainA in one tab, open a second tab manually and type in DomainB (not following a link), then open a third tab and type in DomainC (also not following a link), all three of my domains have the same ECID value assigned to them (without the cross domain service).
Odds are, a high percentage of your users were properly identified across your servers without that change.
If you were to create a Visit segment, that looks for DomainA and DomainB, that should return all visits where the user hit both domains... confirming that the visit didn't start anew when moving from A to B (or vice versa, B to A). You don't need a sequential segment here, simply looking for hits in both domains will tell you if your visits crossed over.
But if users were treated as the same Visit (prior to your drop), that would go against the assumption that visits are now consolidated...
And that would put us back to square one on trying to figure out a drop that corresponded with the release...
