This BBC article does a really poor job of explaining the context of this situation or why fuel would be so much cheaper in Slovenia, so I had to look around. Slovenia apparently introduced fuel price regulations last year (for motorway service stations; off-motorway stations have been regulated for longer), as a means of reducing costs for consumers[0]. These price caps were, in fact, removed a week ago[1], and prices at some stations rose considerably in the aftermath, closer to the Austrian prices across the border.[2] I won't speak to the wisdom of the Slovenian government in trying to cap fuel prices, but however well-intentioned the policy was, it didn't last long in the face of a global energy crunch.
[0] https://sloveniatimes.com/43824/fuel-price-regulation-expand...
[1] https://www.brusselstimes.com/2037901/slovenia-imposes-fuel-...
[2] https://sloveniatimes.com/47009/prices-at-the-pump-up-substa...