For anyone in the U.K. looking for a fix.
I had the same problem and have managed to fix it by buying new bulbs.
The bulbs I had were approx 3 years old LAP (Screwfix own brand) 48mA 9.5W 2700k / 47mA 9W B22. I have replaced them with Sainsburys HOME brand 75mA 8.6W
I also had an issue with some E14 IKEA 45mA 5W and Wilko 40mA 6W 2700K which I have replaced with the Sainsburys HOME 47mA 5.2W equivalent.
The flickering issue is now resolved. I had a bypass installed in all light fitting ceiling roses. So, it does seem to be an issue with certain types of bulbs rather than the Shelly 1L.
In all honesty the LAP and Sainsburys equivalent look identical, but the Sainsburys version is considerably lighter, so internally must be different.