We currently experience, that Google Merchant excludes our discounted products, because Shopify don't include 'effective date' in the feed, regarding the period of the current sales campaign [sale_price_effective_date]
According to our agency, Google crawls our prices, and discover the discounted products. But since our feed from Shopify, doesn't include the information about the period/effective date of the sale, then the Google Merchant center continuosly excludes all the discounted prices.
How do others come around this issue?
That is not correct.
The sole purpose of the sale_price_effective_date is to set a start and end date/time. Meaning if this is submitted, Google will only show the sale price during the submitted window.
If the sale_price_effective_date is not submitted, it will use the sale price when it is submitted and only remove it when it is no longer submitted.
Most likely what your issue is, is related to structured data and or opengraph showing the normal price.
For structured data use: https://search.google.com/test/rich-results to test the product landing page, and make sure the first itemoffer matches the price submitted in Google.
And for opengraph, view the page source and search for og:price