I practice with 3 different Euphoniums, a brass-instrument like a small tuba. One of them - a Yamaha training instrument with 4 valves in a row - has a bad intonation at the natural fifth (upper G) actually the f ... which can hardly be compensated by lip tension or embouchure. I have no idea what could be the reason of this phenomenon as this tone is part of the overtone series.
A perfect air column contained in a perfect, straight tube, plays the notes of the harmonic series.
A real-world instrument, with all its twists, bends and constrictions, doesn't.
Part of the art of designing a good instrument is minimising these imperfections. Or the designer may have taken the attitude 'Ok the open note is out of tune, but it's fine with an alternate fingering'. Try 1+3 or 4.
(I think that's right. We're talking in 'Bb transposed' notation? Anyway, try SOME alternative fingering.)
You think THAT's bad? Look at the overtone series of a church bell. It bears very little resemblence to the theoretical harmonic series. It isn't a simple tuning fork, designed for purity of pitch, it's a very complex resonant system. The 'hum note' may be a major 7th below the 'strike note'! This isn't the place to discuss it fully, just know that real instruments don't have a nice tidy overtone structure!