@rozsazoltan: your last comment solved the problem. However, I still can't figure out how you came to the conclusion that it has to be the size-5 class - which is was.

So what happened? I have recreated a flux component with some heavy customization - if you want to reproduce it, below you find the code that caused the error - and had [:where(&)]:size-5 in a elements class.

How to reproduce the problem: just place the following in your blade file and run npm run build

<svg class="shrink-0 [:where(&amp;)]:size-5 animate-spin" wire:loading="" wire:target="search" data-flux-icon="" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" aria-hidden="true" data-slot="icon">
    <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
    <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
</svg>

Thank you for pointing out that the issue has to be related to the size-5 class.

npm run build still prints Generated an empty chunk: "app". which I don't know why but the process for minifying the css works.