I’m using CF7 5.1.3 with WP 5.2.1 on many sites without problems – I get dozens of emails every day from these sites.
Any problems you are having are likely due to unique issues on your particular WordPress website.
Your problem could be due to a conflict with your WP theme or one of the other plugins you are using.
Switch temporarily (nothing will be lost when you switch back) to a WP default theme & disable all plugins other than Contact Form 7 to see if the problem goes away.
If it does you can switch back to your theme & re-activate each plugin one by one to see when the problem occurs.