Thanks for filing the bug!

Could you please attach the content of the pages linked below, opened from the device that is not syncing (or, in doubt, all):
chrome://signin-internals
chrome://sync-internals

To be clear, my setup is:
    Chrome 73.0.3665.0 running on Windows laptop
    Chrome 71.0.3578.98 running on same Windows laptop
    Chrome stable running on Pixel 2

Tabs from the 73.0.3665.0 instance don't show up on either of the other instances.  I also notice now that the 73 instance doesn't see tabs from the
other two instances (but they see tabs from each other).

The attached files are from the 73 instance.

Deleted: signin-internals.html 15.1 KB

signin-internals.html 15.1 KB View Download

Deleted: Sync Internals.html 123 KB

Sync Internals.html 123 KB View Download

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Tried checking the issue on reported chrome version 73.0.3665.0 using Windows 10 by following the setup mentioned in C#2 as mentioned below.

Setup:
Chrome 73.0.3665.0 running on Windows laptop
Chrome 71.0.3578.98 running on same Windows laptop

1. Launched Chrome of version 73.0.3665.0
2. Signed In then Synced the account.
3. Navigated to few URLs/pages
4. Launched 71.0.3578.98 version
5. Signed In and Synced the same account
Observed the content/pages reflected in the later version's profile, which were searched in earlier version.

@Reporter: Please let us know if we have missed anything in the process.
@mastiz: Requesting you to help in further triaging as chrome://signin-internals and
chrome://sync-internals were provided in comment#2.

Thanks!

Thanks for attaching the details and sorry for the delay.

There are two suspicious thing in chrome://sync-internals:

1. "Server Connection" indicates "Auth error", although signin-internals says otherwise. I think this is red-herring and after talking to treib@ probably due to some past auth error which is no longer relevant.

2. Bookmarks is the only datatype is state STARTING. 

Reporter: could you see if the issue goes away if you disable the flag chrome://flags/#enable-sync-uss-bookmarks ?

Meanwhile, assuming this is a bookmarks-USS issue, assigning to mamir@.

Thanks!  Disabling chrome://flags/#enable-sync-uss-bookmarks did indeed fix this.  Let me know if you want me to run any further tests.

Thank you for trying it out and the update.

Please, do my a favor and do the following:

1- Send chrome://sync-internals in the current working state.
2- Enable chrome://flags/#enable-sync-uss-bookmarks and try again to see if the error is still persistent or not, and also send chrome://sync-internals.

Thank you once more for helping debugging this issue.

chrome://sync-internals's are attached.

With chrome://flags/#enable-sync-uss-bookmarks enabled now, the problem does not recur.

Deleted: Sync Internals - disabled.html 215 KB

Sync Internals - disabled.html 215 KB View Download

Deleted: Sync Internals - enabled.html 167 KB

Sync Internals - enabled.html 167 KB View Download

Thanks a lot for the quick response.
Much appreciated.

Happy to know that the issue doesn't happen again.

I will update the bug with any findings.

The issue doesn't seem to repro(...as per C#9 & C#10) when the flag chrome://flags/#enable-sync-uss-bookmarks is set. Hence removing Needs-Bisect label. Please feel free to add the label back if required.

Thanks!