I would not call it "normal" but I think some of the less scrupolous employers do that. The kind of employer who sack you when you get cancer because it would not be fair on the other employees to cover for you while you're having life saving treatments.
Some companies calculate holiday pay on your average earnings [including overtime] over the previous financial year........so the more you work the higher your holiday pay, but it should never fall below the level of your Basic pay.