Apparently, the stagger overwrite bug is back. This was fixed in Patch 4.6:
After Patch 4.6.4, if you push an enemy knocked to the ground by the Foot Knight ult, they will immediately stand back up:
Source: Peanutchoc comments on Did Large Fish revert the best change this patch?
I prefer Patch 4.6 behavior where pushing them won’t make them stand up.
Linking the old stagger overwrite bug reports for reference:
Summary
Low amounts of stagger reset fallen enemies
Enemies changing to much shorter stagger state if hit mid-air after Kruber charge
FK Valian Charge: its stagger effect can be overwritten and and "canceled" by another source of stagger
State of Knight
Several bugs - knocked over northmen frothers, Bret longsword riposte locking, Saltz red crossbow illusion, HM half a stamina shield bug
Valiant Charge
Enemies can have their stagger cancelled on uneven ground
PS4 Footknight ult knockdown is ignored by enemies occasionally
Mauler Skips Getting Up Animation and Strikes Back