Ok, so Elemetor is loading this CSS:
.elementor-kit-1119 a {
color: var( --e-global-color-accent );
font-family: var( --e-global-typography-accent-font-family ), Sans-serif;
font-weight: var( --e-global-typography-accent-font-weight );
text-transform: var( --e-global-typography-accent-text-transform );
font-style: var( --e-global-typography-accent-font-style );
}
Which is being applied to every <a> in the site. And this: text-transform: var( --e-global-typography-accent-text-transform ); is setting them to lowercase.
Unless you know how to disable or change that in Elementor you will need to add this CSS:
.main-navigation li a {
text-transform: uppercase;
}