I tried with different mod file formats. There’s a bug, everytime when loading the same mod file, the bpm always changes.
Example: i load a mod file, it load with 120 bpm. then i load it again without saving and suddenly it loads with 125 bpm, next time with 80 bpm, then with 150 bpm, etc.
Seems the tempo of some of the mod files is not detected accurately in Renoise when loading mod files.
Can you share a .MOD file (or files) for testing?
When creating a new song in Renoise, it does default to a “random” common BPM like 80, 100, 120, etc. But I’m not sure why this might happen when loading an existing .MOD song… unless it’s a really obscure format that doesn’t have a well defined tempo.
Hard to say for sure!
Here it happens with one of the Pinball Dreams mods:
In other trackers and also mod players this does not happen.
Hrmmm, yeah… Indeed… When I import (and re-load) that one into Renoise 3.4.4 the ZTxx command on the master track of the first pattern is different every time…
Very strange!
Hopefully @taktik can make sense of this one! 
Tried it again after the 3.5 update and the problem is solved. Thanks!