Spectacular Palm (one of my favourites in fact).

Are the fronds dying lower down as in older near the outside and base or are they the new ones from the middle? Old fronds do die away periodically.

This is one of the few Palms that can be considered hardy(ish) in sheltered parts of the UK. I wonder if it is worth trying it planted somewhere?

Brown spots and yellowing fronds often indicate a magnesium deficiency. On the other hand, if it's just the bottom few fronds, they may just be getting old.

The jubaea would be better off planted in the ground, it will grow faster and it's hardiness will increase. I would say it's a mineral/ element dificiency or over watering. Jubaea ARE the hardiest feather leaf palms that we can grow, they are slow growing till about 17-20 years then they pick up growth. One reason for this is root mass.
I have them from 5 year olds to about 45 foot high in Yorkshire.