Sounds like a loose connection or possibly a bulb that is close to giving up the ghost.

 

Still under warranty?

 

My 2018 had an issue a couple months ago with the trunk light not turning on. The dealer replaced the trunk latch opener and it’s all good now.

 

Yeah thought it was bulb so I changed it with an LED bulb and it acts the same way, guess I gotta try to disassemble the latch. Bought it used so I gotta check with the dealer if the bumper to bumper is still in effect

 

There's a light in the trunk my 2018 apparet don't work then, just figured there was no light

 

Some trunk lights have an on/off switch in the trunk on the light itself. Could be it's just turned off there.

 

The 2018 Sonata no longer has the valet button that the previous model years have. The service writer at the dealership was well aware of the issue with the trunk latch openers for these cars and he even made a joke about it. Most dealers will have the part in stock, and there are several outlets on eBay selling the OEM part with numerous reviewers. To be frank, it’s very common for this part to break. Luckily it’s an easy fix and not very expensive if you’re out of warranty.