Try remove the margin of the cards and use
gap property instead.
gap will literally create gaps between flex items.
@joanneccwang I held off doing that because only the Technology Preview release of Safari supported gap with Flexbox. But I just checked caniuse.com and Safari 14.1 finally supports it as of last month on April 25th, 2021.
For me, that might be the biggest reason to update to the latest version of Big Sur.
Thanks for the tip!