10.13.4 = 1 fewer monitors on my MacBook Pro mid 2012 (non-Retina)?
Your problem is due to a bug in the DisplayLink driver used by your converter.

DisplayLink are aware of the problem,. They have issued a beta of a new driver that actually doesn't fix the problem. You can view their note on the problem at http://www.displaylink.com/downloads/macos It doesn't support multiple screens on multiple displays.

They have a mailing list supposedly for updates but have failed to notify the mailing list of the last two updates so I wouldn't bother.

I also wouldn't blame Apple. Both Apple and a number of 10.13.4 beta users (including myself) advised DisplayLink of the problem quite some time ago.

Update: DisplayLink have issued a second 4.3 beta that doesn't fix the problem. They have also removed the 4.1 version from their site. (4.2 has never been seen.)

(I will update this answer when DisplayLink put out a working driver.)