In 1.13-pre6, changes were made to the fonts in Minecraft which affected obfuscated text.

The bug

Since 1.13-pre6, obfuscated text now uses a severely limited character set (looks like just the ASCII character set) whereas it previously used a rich character set of accents, letters, symbols, and other peculiar characters.

I have uploaded a video which shows the effect in 1.13-pre5 and 1.13-pre7. The video is at 25% normal speed but you can also view it frame-by-frame or test yourself.

How to reproduce:

On either version of Minecraft, enter:

/tellraw @a {"text":"abcdefghijklmnopqrstuvwxyz1234567890","obfuscated":true}

Note

This bug may also relate to MC-132664, which covers obfuscated text not randomizing.