SEO Related Issue: Choose Different Canonical than Users

Please share your suggestions for why this is happening with our website.

Our website is e-commerce.

Our number of index pages are decreasing day by day. We are getting them indexed manually but after some time it's getting deindexed again and it's happening since the last month only.

For an example, our this page /sunglasses/gender/men is no indexed now. Along with this we are having some canonical issues as well. We have given the different canonical url but google is taking different canonical url different canonical url. Given the reference below.

For Example: We have given self canonical to this page( /sunglasses/gender/men ) but google taking (/eyeglasses/gender/men)

Note: We have checked our content also. Both Pages content are different so why it's happening?

Create a single sunglasses category and re-direct the one which doesn't give a lot of traffic.

Google bot can consider different canonical IF they feel that page is more relevant.

Do a comprehensive technical seo audit to find duplications and other factors. Your GOAL should be to utilize the crawl budget efficiently and make sure Google is not wasting resources on bad pages such as 400's and 500s error.