Some CSS was added in app version 3.6.1 which has changed the colour of the arrow on an ion-select that doesn't have a colour specified.  Previously this would use the primary colour (orange) but it is now grey.

The SCSS file for this element contains the following code:

ion-select {

   ...

    .select-icon .select-icon-inner {

        color: $core-select-placeholder-color;

    }

    &.select-disabled, .select-icon .select-icon-inner {

        color: $text-color;

    }

    ...

I'm not a SCSS expert, but as I understand it this will result in a CSS file where the second rule will always override the first for ion-select .select-icon .select-ion-inner elements.
I'm not sure which of these needs to be removed or made more specific, but I'm guessing this isn't the intention.