The common wisdom behind the behavior is that some mod effects are perceived as providing attenuation, though my experience with various pedals and racks over the past 20+ years do not support the degree of compensation that Line 6 has implemented.
I think the best thing would be to post a feature request to allow the user to choose between the stock gain adjustment and unity gain at a minimum, if not the ability to set the level of the block.
Workarounds are just that. They require burning blocks on volume adjustments that often result in unwanted pops when toggling on and off; or require configuring parallel paths, or using outboard gear. Back when I was more active here I think I read about them all. The most common one is to run these effects before distortion/compression/amp models. For me this is also unacceptable. I have always preferred to affect my compression/distortion rather than compress/distort my effects. If you like it the other way I say hats off, but I don't see why one should have to compromise and use a different a configuration, when the one that is sought can hardly be classified as non-standard, especially when the fix really amounts to little more than tweaking a single register value. Were those rack models (Tri-chorus and Dimension) stuck at the front of your signal chain back in the day? Of course not!
Anyway, pardon my soapbox preaching. I've just read too many responses from people who will tell you there is no wrong way and then later criticize you for not doing it right. Personally, I'd rather see improved flexibility, rather than suggestions that amount to little more than compromises and workarounds.
Again, if you would like to see the current behavior changed or have a new feature implemented in order to satisfy the greatest number of people, please consider soliciting Line 6 through their request link. Please also do the same for the M-series (M9 in particular).
Thanks or your time and consideration.