This site is on Pantheon, but it has been there all the time. Maybe this problem occured after updating WP 5, I’m not sure.
I’ve checked the theme and plugins, but I didn’t find any conflicts. And the odd behavior is same with the default theme and only WCPF7 plugin on.
Theme is PressCustomizr (Child) and Plugins are:
contact-form-7 5.1.1
contact-form-7-to-database-extension 2.10.24
flamingo 1.9
google-analytics-dashboard-for-wp 5.3.7
log-emails 1.3.1
wp-native-php-sessions 0.6.9
responsive-lightbox 2.0.5
sendgrid-email-delivery-simplified 1.11.8
swift-mailer 5.4.3
sitepress-multilingual-cms 4.1.3