Has there been a change to how map service transparency is handled?

Hello.  I have several map services that have semi-transparent layers.  In the Essentials site setup, in the map service display settings, the Image Format combo box is left blank (I've never filled it out).  The layer transparency works just fine.  These are dynamic map services.

Now however, if I add a new map service that has semi-transparent layers, the only way I can get the partial transparency to show is to set the Image Format to PNG32.  Why is this?  Did something change with the last version?

I'm using GE 3.15.1/SV 1.10

Thanks,

Chris