I am afraid stefan s is right. That is the common experience, the LED flashers having more or less the double frequency. At least here there are no MOT problems with that, but it looks a bit odd. That is the reason I did not want LEDs there, have them everywhere else exept the mains (too expensive!) I do not know if there is a LED flasher with a built-in resistor, you might try this company:
https://www.bettercarlighting.co.uk/If you have clear indicator lenses and want a colourless, not amber bulb to avoid the "fried egg look", there are ordinary incandescent amber bulbs with chrome on the outside. They appear colourless when not lit up, amber when lit. It is what I have done, works ok.
