This phenomenon is mostly seen with all lead-free alloys. Gas escape to the outside causes bubble formation on the solder joint surfaces. There is no definitive reason for this phenomenon, although some suspected causes include:

Finish on the pads: This is more visible on ImAg surface finishes, especially those with high organic content.

Flux trapped in volatile materials.

Poor coating on PTH.

Lead-free has a lower density than Sn/Pb so any volatile gas is less buoyant in lead-free. (Lower power pushing the gas out)

In Europe, there is currently no indication of substances to be added to the banned substances list, other than those included in the RoHS directives. JEITA recommends that lead-free alloys contain silver. Because of the need to complete the desired wetting characteristics, silver is a critical additive.