Unable to send links in Teams chat - need help finding cause for Enterprise customer

** Problem statement:
Large amounts of Enterprise users cannot send hyperlinks in Teams chats. If they include a hyperlink in chat, it returns an error "FAILED TO SEND". Client has deleted the cache for these users and the problem persists. Teams Version 1.6.00.29964 (64-bit).

 

** Workaround:
Whenever a user DISABLES the “OPTIONAL CONNECTED EXPERIENCE” this problem seems to go away. This has been tested with more than a dozen users, and simply adjusting this FIXES the problem.

** Details:
Feature appears to be turned on by default.
Turning off manually fixes it per user action.
Customer does not see any policy (GPO) in the M365 Apps admin center for this. Actually there are zero policies in that screen.

** Question:
Microsoft support engineers have tried but have not solved this issue. Any ideas on what the cause could be here??