Did you check "Google Signals" data collection is being enabled (located in Admin > Property column > Data settings > Data Collection).
Guide: [GA4] Activate Google signals for Google Analytics 4 properties
Google Signals is a feature in Google Analytics 4 that collects additional data about website visitors, such as demographics, based on their Google accounts.
Enabling Google Signals enriches your reports with demographic information and helps identify website visitors. However, it can cause data thresholding, where rows with small numbers are hidden in reports (including purchase events as I know).
[GA4] Data thresholds
If the client doesn't need demographic data or retargeting with Google Analytics audiences should disable Google Signals to avoid thresholding and maintain accurate reporting.
Another way: Switching to device-based reporting identity can also unlock hidden rows with small numbers but sacrifice user accuracy.
you can see more explanation in this video: https://www.youtube.com/watch?v=nZUvxKQGfuA