Google does not indexing data from api on page
When you say the page is indexed without data fetched are you talking about the rendered HTML that you can see using the URL Inspection tool from Search Console?
It's hard to make an assesment without knowing the specific URL you are talking about. Depending on how long it takes your server to respond to the fetch request, Googlebot might consider killing the connection.
However, I would say that the testing tools from Google, like the URL Inspector tool from Google Search Console, have lower times thresholds than the "real" Googlebot because the testing tools prioritise to give you results quick. Hence, it might be a limitation from the testing tool.
If the data that you're fetching it's really imporant it might be worth considering dynamic rendering for Googlebot. Serving a pre-rendered version of the page to Googlebot and let the users have the real experience.