Sympathies, we had a Polo from new which, for quite some time, exhibited similar symptoms, not apparently believed by the local franchised dealer. They simply plugged it into diagnostics, which reported all was well. “Computer says ‘No’”. So they’d send us off reassured all was well, when it never was. They did replace random bits’n’bobs under new car warranty but they were just guessing, as the computer wouldn’t tell them what was wrong, and they never fixed it.
Trouble is, if the dealer doesn’t believe you, and doesn’t want to believe you, will the dash cam persuade him? He’ll see the car slowing, or stopping - but will there be any evidence of a fault, or could he claim it must be driver initiated?
In the end, with our Polo, we took it away from the franchised dealer network and had it serviced by a well regarded local VW independent specialist, who was not totally dependent on computer diagnostics. Fixed, at first Attempt.