Huge Bubble When Toilet Flushes, Why?
The hole in question is a siphon jet.  It concentrates flow from the tank, jump-starting the siphoning effect.


If you look close (I’m not suggesting you do), there’s a siphon jet hole (see letter G on diagram) in the bottom of the toilet bowl. It’s hard to see unless you put your head in the toilet (For the record, I didn’t do this). When you flush, water shoots out of this hole to fill the drain pipe. This creates a siphon.
When I investigated, it was plugged. Eureka! I cleaned it out and once again heard that perfect flush. -tomlaforce.com

See (acid), How to clean under the narrow toilet rim to restore full flush power

When you flush the toilet in the diagram, the chamber directly above the X fills with water. This chamber is connected to the same passage where the air is, above G. It's also where all the holes are and once they fill with water, the air is forced out through G.
A noticed change may be a clog somewhere but perhaps as the guest toilet, you never realized how poorly it preformed out of the box. IME this is typical of low-flow toilets but also of every toilet that hasn't been flushed in a while.
You may be interested in adjusting or installing a chain float which will hopefully allow enough water to flow after the bubble pops, to create a siphon.