Yellow doesn't like to be encased?
 

Just made batches of encased spacers over the past two weekends. Single non-reactive inner color with a thick clear encasement.

Almost 20% of the yellow cracked. One at a time over a period of a week.

I don't remember yellow having issues this way and the yellow is at least 8 years old.

I think the hot colors; red, orange and yellow - are bad about being encased. Try a clear core, yellow, then encasement if you can get it all in the spacer maybe?

Huh.

I'll give it a go. Makes me encase twice. Ouch.

Maybe someone else will know of a yellow that doesn't need it.

Eileen is correct, and some yellows are better than others. Laura Sparling has a blog with some fave yellows for encasing - you have to look though, it's not all in one post, and I think at least one or two were discontinued. ouch.
You might try enamel over a clear core. It would go faster and would be more stable.