I have three clips in parallel that are all routed to master, no sends or fx. The Master gives me a peak value of -2.2dB. I bounce them to a single clip using "what you hear" preset and change the output to mono and no dithering (all source files are mono as well). Now I'm getting a peak value of 0.8dB so it's clipping. All the tracks are routed straight to Master without sends or fx. What am I doing wrong?

Could it be one of the cases discussed in "Mono audio clips may be increased by 3 dB in certain scenarios" on this page https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Playback.19.html
 

I have some stereo clips (original clips that I've muted) and mono clips in the tracks. So I guess it's that then. Using "split mono" seems to give same values after I delete the other clip.

Joe,

Usually when my bounce is getting louder evenly, meaning for the whole duration, it is because I'm also bouncing in a previous bounce.

So, recording mixed with the recording= the recording plus 3dB.

Any chance this clue might help?

It's not so much about the channel config of the clips/tracks; it's that you're bouncing a stereo bus to mono which will add 6dB when both channels are identical (i.e. 100% correlated) or approx. 3dB if they differ (uncorrelated) as appears to be the case here. You'll need to lower the Master bus 3dB to compensate for summing two channels to one.

Good points. I think it was caused by mono setting instead of mono split. The source material is all mono so the overall increase should be +6dB if it's doubled. The mono split gets the job done so I'll just stick to that.