The decision of which of the two options (APU/engine bleed versus ground equipment) to use to condition the cabin air at the gate has nothing to do with avoiding fumes. Some airports provide the equipment, some don’t, sometimes it’s not available. By the time engines are being started, ground AC equipment will generally have been disconnected and air will be supplied by bleed from the APU or an engine that was already started.

The patterns of whether and when ground AC is used haven’t really changed industry-wide at any recent time, and certainly not around the pandemic, so I don’t know where the idea of this “period of time” comes from.

Maybe your travel patterns changed, or the ground AC equipment availability/policies changed at your frequently-visited airports. Or maybe the aircraft you are flying on were parked during the pandemic and then the reactivation maintenance (esp. engine washes) was not thorough enough, causing fumes at startup.