book_unit_form_submit doesn't use hook_rooms_price_modifier_alter to load the $price_modifiers

I have a module that implements hook_rooms_price_modifier_alter ::

/**
 * Implements hook_rooms_price_modifier_alter().
 */
function pricing_rooms_price_modifier_alter(&$price_modifiers, $booking_info)
{
	// This modifies all Units to have a price of 100
	$price_modifiers['a'] = [
		'#type' => ROOMS_DYNAMIC_MODIFIER,
		'#quantity' => 1,
		'#op_type' => ROOMS_REPLACE,
		'#amount' => 100
	];
}

This shows the correct price in the search results. When I book that unit, the basket shows the wrong price. It has not called my hook_rooms_price_modifier_alter function, so the price modifiers are not applied.

Viewing my unit in the search results shows the price to be £100. Clicking on 'Book Now' then adds it to the basket with the wrong price.