When you have a text field in the Markdown format and want to preview it, the Help button is disabled. When it is, the label of the button becomes not visible.

This is due to the rules used on tlp-button-secondary when they are disabled:

    &:disabled,
    &.disabled {
        @include tlp-button-disabled(var(--tlp-neutral-light-color-lighter-30), var(--tlp-neutral-light-color-transparent-40));
    }