Do this. Empty the tank leaving the valve open. Fill the bowl to within 1" of the rim and flush. If it burps then you have a blockage in the pipe between the toilet and the tank or a mound.
If it does burp keep doing these super flushes until you clear it.
Report back. I have the same offset in my down pipe and the pipe joints could be causing "things" to hang up in the pipe. Two or three gallons of water dropping in a 3" pipe is a good force to clean out things. Might wear your rain gear when doing this the first time!
Chris