When sweat, aluminum salts in the antiperspirant, and washing detergents come together, they often leave yellow stains on white shirts.
To avoid this, you could switch to a deodorant that doesn't contain aluminum or use less of the product to reduce the amount of dust. It's up to you, but this can help keep things from darkening.
It can also help to change the way you apply for things. Don't put on too much deodorant; let it dry completely before dressing.
Shirts can stay fresh longer if you wash them right away after wearing them. Before cleaning, you can help get rid of stains by treating the area under your arms with a stain remover or a mixture of baking soda and water.
Soak shirts in a mixture of white vinegar and water to get rid of stains that won't come out, or use a paste made of baking soda and hydrogen peroxide on the spots before washing.
If Rexona keeps giving you trouble, you can call their customer service line at 1-800-298-5018 or use the contact page on their website (https://www.rexona.com).
Your voice matters. If the issue persists, share your experience and consider filing a complaint on Xolvie to make a difference.