While in tactical pace, it is impossible to move while sighted in. However while in jogging pace, it is possible to have sights up while moving. These 2 animations must have been accidentally switched.
While in tactical pace, it should be possible to have the weapons shouldered and sighted while moving. While in jogging pace it should not be.
Switch to jogging and tactical pace and use sights.
I would describe the issue differently:
(Here I use "ADS" to mean aiming down the weapon's iron sights or optic; its equivalent in the Arma 3 keybinds is "Optics" but I choose to use "ADS" for brevity. Likewise, I describe the default movement speed as "run", not "jog", and "unsighted" view -- that is, not ADS and crosshairs visible if enabled -- is considered the default view in this explanation.)
When moving without ADS the paces do not present the error; likewise, ADS while walking does not present the error, and ADS while running correctly forces the character into combat pace for the duration of ADS.
The error is that when the player manually toggles combat pace from the default unsighted view and then aims down the sight, the character's movement speed instead changes to run and there is a different "weapon bobbing at bottom of screen" animation than when normally running. From THIS state, "de-toggling combat pace" causes the character to return to combat pace and thus enter "aiming through sight" view (ADS) while leaving ADS will cause the character to return to the normal run. HOWEVER, from the aforementioned state where the character runs with the different first-person-view animation, clicking the ADS button (which would NORMALLY -- and in the prior stable build did -- just cause the character to leave ADS) causes the character to 'revert' to unsighted view and combat pace.
TL;DR: The current situation essentially "reverses" the ADS/running/combat pace dynamic from the way that it was in the prior stable build, where if you manually toggled combat pace from the default non-ADS view and then ADS you would CONTINUE moving at combat pace UNINTERRUPTED, while in the current situation attempting to ADS instead forces the character into running speed with the different "weapon bob" animation, and attempting to "de-toggle" combat pace instead CAUSES combat pace-movement-with-ADS.
At this time the only way to simulate the prior situation (with the prior stable build) is to be simultaneously pressing "combat pace toggle" and "optics"/ADS so that the effects counteract one another.
The error is also present on dev build 0.59.105634 as well as the stable build.