Tested with:

• WorldGuard 7.0.2-SNAPSHOT;bf2593e
• WorldGuard 7.0.3-SNAPSHOT;2029
• Spigot 1.15.2 (latest)
• Paper-166 (minecraft 1.15.2)

I have created a region with flags and even if the deny message is showed, player can place blocks. But they can't break them, maybe a PlaceBlockEvent bugs, there is nothing on the console logs... :/
I don't have an other plugin with region system or prevention system.

Flags used:

• interact: allow
• break-block: deny
• place-block: deny

If the message shows up WorldGuard has prevented it. It’s most likely another plugin overriding WorldGuard.