In both the Western countries I’ve lived so far (France and Italy, but my guess is the list is much longer), this wouldn’t be possible.

The landlord could at most adjust the rent to inflation.

The only wag he/she could ask for such a steep increase is by making a brand new contract with someone else, but for that purpose the current contract should be terminated, which isn’t possible if your occupants are paying their rent, unless he/she wants to re-take possession for selling the apartment/house or to live in it. If they fake the repossession and actually end up renting again to an inflated price, the new contract is deemed void and the previous occupants could be reinstated.

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