Rather than setting the slide to have flex, create a div inside the slide with a setting of “position: absolute” make it fill the slide “100% width and height” then set this same div to have Flexbox, then place your text inside.
First, in general dont add layout style to the core slider wrapper (You will break the layout) - this is not a bug this is the idea in any Slider plugin out there (The core styles are important for things to work).
Center horizontally text/image
its very easy. You can use text-align: center; - simple without extra styles. Example.
Center Verticaly
This is more tricky but with flexbox is very easy (Add the flex to extra wrapper inside slideX not to the core elements wrapper).