On Opera mini, the problem isn't with the opera mini engine, when a site is broken not as expected, you should debug it yourself, especially if it's your site.
2nd, extreme data saving isn't intented to make a website beautiful, it will fuck up some site, and seems you use a lot of 3rd party CSS library that's not supported by opera mini. Ask people to use high data saving mode so it will shown fine on the device. Just it.
It's been a long time public secret that and heavy 3rd party CSS library will broken on opera mini, and if you want to your site work on extreme data mode, then check the browser agent, if it's opera mini, use a working css and JS that's justified to work with compression on opera mini. just it.