I have read the other posts about this issue, but mine seems slightly different.

The toilet on the ground level on a slab loses water from the bowl slowly after a flush until it is empty. Also it flushes fine. The other toilet in the house on the second floor is fine. The problem bowl isn't exacerbated by flushing of the other toilet or using the washer. There are also no signs of leaking of water on the floor around the toilet and the toilet is secure to the floor.

If it were a venting issue the second floor toilet would be effected too, correct?

Both toilets are at least 6 years old and are gerber brand.

The lids are down so it's not the dog. I also plunged the problem toilet and it didn't seem to help.

Is it a vent issue or do I need to go toilet shopping?