I've noticed this, too, but I'm not sure if it's been intentionally done. I have tested it while logged in and also in incognito mode on the desktop and in the app.
Lightning used to be shown by a lightning symbol, a 'flash' when it occurred, and a noise. I no longer get the noise (even with sounds enabled) or the noticeable flash, and the lightning symbols are otherwise very easy to miss -- especially if zoomed out to a wide area, as the symbols become tiny.
Also, in times gone by, lightning strikes would leave a 'scorched earth' brown symbol behind for a while, so we can track the movement of storms over the ground. This seems to have also been removed, unhelpfully.
I appreciate the team's hard work on Windy, but as a long-term subscriber, I think the most recent 'improvements' have been anything but. It's a shame.