Facing the same issue. Looks like jetpack and WordPress 5.3 have some conflicts. Disabling jetpack resolves the issue for me but looking for a useful solution. Also, I’m facing the issue on jetpack 7.9.1. And @macmanx we don’t have wp-spamshield in our site. And we are not facing the issue in all of our sites. It is quite random. Tried rolling back jetpack haven’t seen any success with that. Cleared all cache, front and back end; resulted in nothing. Disabled w3tc; resulted in nothing. Quite annoying issue. But in Incognito mode in chrome it works flawlessly.