Hi,
Could you please clarify if you are using the standard wordpress comment form or a third party plugin?
If using the standard form please complete the following steps in order to detect the problem and find its solution:
Deactivate all of your plugins except reCaptcha and also check if the problem is still present. If it is not, activate the plugins one-by-one, meanwhile checking, after which plugin activation the problem appears again.
Report the result.