AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Long Formulas
Mathematical Composition

Long formulas may be given in 2 or more lines by breaking them at operation signs outside brackets or parentheses and keeping the indention the same whenever possible (some formulas may be too long to permit indention). If lines begin with an operation sign, they should be lined up with the first character to the right of the operation sign in the line above.

However, if a formula loses comprehensibility by being unstacked and broken up and/or if it fits the width of the column, it is preferable to leave it stacked.