I've noticed this aswell. You are currently unable to perform sprinting attacks with LMB + SHIFT. You can perform the "shorter" version, but not the original version.

Yeah there's now an army of freshies exploiting this broken mechanic, it's hurting the game

I wanted to add more context to this post. You can still do the "running power attack" however since 1.27 it's different.
Previous to 1.27 it showed a running HEAVY attack, which was slow and had a range of 3m~ in front of you. (Done by running 8 steps and pressing LMB)

1. Since 1.27, you can still perform the same running power attack, however it comes out as a LIGHT attack, which is VERY fast. Done by running 8 steps and pressing LMB, this attack appears to be a light attack, but will still stagger and go through a player blocking. This attack also still has the added 3m~ range in front of the attack. (depending on fist or melee weapon)

2. Since 1.27 there is also another attack which is similar to the running attack but can be done instantly. This is done by sprinting for a second and then pressing RMB and LMB instantly after. This has a HEAVY animation and will stagger and go through a player blocking.

Suggestion: Revert the running power attack to be a HEAVY attack animation. This at least allows a counter to the running light attack spam meta that has happened since 1.27.

Will this fix be in a hotfix, or do we have to wait for 1.28?

In T189430#2779252, @Nate_LapT wrote:

Will this fix be in a hotfix, or do we have to wait for 1.28?

Should be resolved in the upcoming patch to 1.27