Can confirm in 18w19b vanilla server.
Windows 10 Pro 16299.431 x64
Java 1.8.0_171-b11
Glad I ran a world backup. I hope the team can come up with a fix.
Confirmed in 18w16a. Was AFK from my server and came back to a sea of fire.
Rain manages to stop it slowly but surely, barely around the trees though.
The fire doesn't actually seem to disappear, judging by the lightspots that continue to exist after extinguishing fires with rain. This also causes big lag on the client side; the higher the render distance, the worse it gets.
Also confirmed for 18w50a, taming a wolf (first animal I tamed in its world).