I don't know why it behaves that way. These are just suggested workarounds rather than straightforward fixes.

Back in the good old days before the Equation Editor, you could create equations with an Eq field and they stuck together. You could always use that which still works.

Another alternative is to encapsulate the equation inside something that doesn't naturally break across lines. have you tried putting your complete equation inside something else that doesn't break. For instance you can insert your equation inside a superscript entry box using the equation editor and that equation will be smaller but will not break at the end of a line. Interestingly, putting the same equation inside the non-superscript box will not stop it line breaking so you need to put a space in there to hide that box.

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia