If there is a bad display of the line thickness on the printout of the Printing Tools (too thin line thickness on the printout compared to the display in the map image) when using WMS services, then the following workaround can help.

 

Is the following parameter set to "True" on the WMS service in the map collection of the project configuration?

If yes, then this parameter should be disabled.

This parameter does the following:

Determines whether a larger image is requested for a high-resolution printout. Since WMS supports only screen resolution, using this setting may result in different display between the screen and the printout or in changed symbolism on the printout.

 

Alternatively, print with a slightly lower DPI number (just not high-resolution and less DPI - adjustable in the print template in the project configuration):