Mine always shifted the same in regen or not.
Only similar 'bump' mine ever had was the one that is commonly associated with fuel slosh.
And that only occurred with a certain combination of decel rate and fuel volume in tank.
Downshifting hard enough to 'chirp' the tires makes me there is a physical problem with the transmission.
If not, there would have been many many complaints posted about the 'chirp' between late 2010 and now.
2018 GMC 2500 L5P CCSB 4x4 Denali - Mineral Metallic; previous '11 LML, '10 LMM and 04.5 LLY