How to change frame rate of MJPEG segments with/without re-encoding?
It's a warning, not an error (if it would be an error, FFmpeg would abort).

“Non-monotonous DTS” means that the DTS (Display Time Stamp) of some frame is less than the DTS of the previous frame. 

This situation may cause many problems (with playback, seeking, etc.), so 
FFmpeg fixes timestamps by rewriting some of them.

Here are your 3 warning messages: 


[...] Non-monotonous DTS ... ; previous: 86, current: 73; changing to 87. This may ...
[...] Non-monotonous DTS ... ; previous: 87, current: 74; changing to 88. This may ...
[...] Non-monotonous DTS ... ; previous: 88, current: 75; changing to 89. This may ...



in a graphical form: