These are the steps to consistently create a waveform that is displaying the content of the wrong buffer:
1. new patcher
2. new object: buffer~ xxx 3000
3. new object: waveform, open inspector, set buffer object name to xxx
4. save patcher: untitled1.maxpat and close
5. new patcher
6. new object: buffer~ yyy 5000
7. new object: waveform, open inspector, set buffer object name to yyy
8. save patcher: untitled2.maxpat
9. select all, copy, close patch
10. open untitled1.maxpat
The waveform previously set to xxx now will be 5000 ms length, and opening a file to the buffer yyy using the read message will have both waveforms display the content of yyy.
Now these steps don't seem to work in 5.1beta. Either this issue has been adressed, or by chance this bug is not anymore the result of doing things in this order. If the latter is the case, hopefully this gives more insight. If it is the former, then happy me.