I've had this theory for a while, but the in-game models have given me a bit more proof. The images are primarily to illustrate this evidence, but are certainly a bit iffy and don't demonstrate my points the best.  
The idea itself stems from three primary things: D.Mon's remeching animation, her mech's gibs, and her Fusion Cannon.  
As opposed to how D.Va calls in a new mech and "spawns" it (almost certainly representing a new mech dropping to her position), D.Mon reassembles pieces of her mech around her to call it in. This is the weakest point of evidence individually, but it is still the backbone of this concept.  
The gibs of D.Mon's mech is the primary evidence, as they are surprisingly intact compared to D.Va's mech when being destroyed. D.Va's gibs are blackened, spark, and contain several unidentifiable scraps of metal, whereas D.Mon's gibs could pass for just being disconnected parts on its own if not for the smoke and context.   
Her Fusion Cannon also points to the general intent of this design, as it is clearly intended to be detached from the mech when rendered inoperable and acts as a considerable threat. Contrast this with D.Va's pistol, which is clearly meant as a last resort and seems rather pathetic compared to the Fusion Cannon.  
With this in mind, it would explain why D.Mon is so exposed in Beast and the reasons behind each of my points; it's because Beast is intended to be scuttled even more than Tokki, and to this end D.Mon likely abandons it shortly before significant damage is inflicted so that it may be recalled and repaired later.   
As for why this is, I suspect it is primarily to decrease the downtime between repairs and allow D.Mon to take bigger risks in battle. It's not exactly like they're throwing her to the wolves, supposedly she may have prior military experience and the weapons in her room indicate some form of combative ability outside of her mech. It's less ideal for combat, but better for practical purposes.  
I do wish to mention that this may just be me making random connections based on the devs minimizing visual clutter, but I still think it's a neat head canon that explains the design and that it's something worth sharing. Hopefully you think it's interesting.

Beast seems to have been redesigned with mid-combat scuttling in mind