Hello andibo,
Thank you for using Apple Support Communities. It's my understanding that you're intermittently prompted to send a message with effects, even if the recipient is doesn't have an iOS device. I can see how this could be concerning. I'm happy to help.
Enabling Reduce Motion will prevent you from receiving message effects, but doesn't impact sending messages. If you haven't already, make sure that your iPhone has the latest software version.
Update your iPhone, iPad, or iPod touch
If the issue persists, force Messages to close and restart your iPhone.
How to force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone
Let us know the results. Please include any other troubleshooting steps that you may have tried.
Have a nice day!