I have a Diamond X-6000A triband antenna, and its SWR curves are 'wavy'
on VHF and a little on UHF, not the smooth symmetrical log curve one
might expect. It's likely the interaction of the other band resonances,
but nothing to lead me to believe it's broken.

I like the suggestion about trying a good quality dummy load on it (or
perhaps another analyser on the same group of antennas) before you
condemn the thing as junk.

Good luck!

B