Why do you think it is our plugin problem? You installed the optimization plugin (I don’t know which one exactly) and it loads styles using JS. For example, I see this optimization on all styles and maybe something similar is not allowing our plugin styles to load properly:
I think this flashing happens because of some sort of optimization. Try to deactivate 3rd-party plugins one by one to determine which may conflict. Also you can test it with standard theme Twenty Twenty Three.
Thank you for disabling the optimization plugins, now I can see that our plugin adds style fallbacks instead of regular <style> tags in the head.
Usually, this happens when there is no possibility to parse block output properly. Most probably something is conflicting in the Blocksy theme. We tested it with popular themes like Twenty Twenty and Astra and everything is working fine. Looks like Blocksy renders the content in a different non-standard way.
I have added a task in our issues tracker, we will look at it in the next plugin update.