Hi Jennie,
We add automatic 301 redirects to all category URLs and that is what Google suggests to do when there are changes in URLS.
Changes in URLs are not a reason for ranking loss (unless you don’t redirect old to new URLs).
All WordPress websites have “category base” and “tag base” within their URL structure, we added it to comply with the default WP permalinks system. If that was a problem, no WordPress site could be SEO optimized, which is obviously not true.
If Google saw value in your page without the word category in the URL, it will still see value after adding it. Unless content on those pages changed radically, it’s not realistic to think google would drop your rankings just because of the extra word “category” in your URL.
For example:
This URL is ranking 3rd in Google for the KW parks in Perth: https://www.buggybuddys.com.au/things-to-do/category/playgrounds-and-parks-in-perth/
This one 8th for Christmas light: https://www.buggybuddys.com.au/things-to-do/category/christmas-lights-perth/
apple picking Perth in 1st position:
https://www.buggybuddys.com.au/things-to-do/category/fruit-picking-perth/
All these URLs have the word category in it:
Your traffic loss is definitely not related to the extra layer in your URLs.
I’ve seen websites with a lot more authority than yours, losing traffic since the end of September which is when Google started testing their latest Algorithm including BERT. https://www.blog.google/products/search/search-language-understanding-bert/)
Did you check in your Google Search Console to establish exactly from which KW and pages you are losing traffic?
Let me know,
Thanks