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Why would you assume Teams and not the browsing habits of everyone working on an ad for this company?
There's got to be like an order of magnitude more chance it's just people in your org doing internet searches about said company and you guys are already linked because you work for the same company.
Also, if this company is paying you to make ads, it's just likely they're also simply pushing more ads out via other means.
Also, how do you know it's not LinkedIn? Someone in your team added them to a list of clients or chatted about them (does Linkedin have DMs? I have no idea)
You can reduce obvious 'chat' data sharing by doing things like disabling gifs and link previews: https://learn.microsoft.com/en-us/microsoftteams/teams-privacy-oce-overview
Microsoft needs to be a little bit less obvious with their spying on Teams, today LinkedIn send me a job ad from the company whose ad we are currently working on.