Don't know if it helps...Also upgraded recently to 18.3 and faced with a similar
The thing drained HiDPI and Cinnamon 3.6. LM now has the ability to automatically scale the size of the Windows\fonts\icons under the resolution of the monitor. To solve adjust the resolution of each monitor by hand. If you do not help, here is a review article
, then naguglit about tuning the X server.