I agree, in that case, if you do not want to use a paid service is to contact Shopify, they are the app developers, which maintain the free app you currently are using. Nobody on this community will be able to fix the app itself, only Shopify can do that. The more people complain to Shopify, the more likely they will do something about it.
I also recommend you check the reviews of the current app you are using, you will see lots of negative reviews.
Because I service clients that want to achieve goals (sales and profit), I personally do things differently, when I know an app is not capable of achieving goals, I use a different app that can. Because in the end what is important for my clients is sales and profits.
If I can not generate profit with the free app due to limitations, bugs, etc. Then I will use something else. Yes, profit will be slightly lower, but in the end, there are more sales, because I am able to set up a much better feed. Of course certain recommendations I make, will be outside the profits of my clients, but that is very rare, as all merchants have to spend to make money.
Same with Google-related issues, if Google is unable to achieve something I need for my clients, I find other ways to get the results I need.
From 12+ years in business, I have quickly found that I can give my clients better results by using tools that work, rather than trying to force use free tools, that don’t work.
Again, I understand from your perspective as a merchant, from my business perspective, I make decisions that help my client’s overall goals.
Having issues with the feed need to be resolved. Regardless of what app you use.
You can if you want something absolutely free, is to create a data feed manually, using a spreadsheet. You can follow the guide here:
https://support.google.com/merchants/answer/7052112?hl=en
https://support.google.com/merchants/answer/160567