Here are the conclusions from my further testing:

I removed all explosion particle effects that were triggered at the same time as the shockwave script, and tested again — the issue still occurred.
This indicates that the problem is not related to particle density or FPS drops.

In contrast, when I executed the shockwave script on its own, the issue did not occur.
This suggests that the problem lies in how the script is executed, not in the script itself.

I went back to my mod and found the following in the file:
WarFXPE\ParticleEffects\config.cpp
Look for the class:
class TT_USAF_MOAB_explosion_effect_sqf_script_trigger
(You can use a text editor to search for it, as this file is quite large and hard to navigate manually.)

This is the special effect where I used the beforeDestroyScript parameter to call the explosion script.

I believe the issue is caused by how the beforeDestroyScript parameter invokes the SQF script.
I suggest checking how the engine internally handles the beforeDestroyScript parameter — specifically, whether anything is done before the script is actually executed.

After testing step by step, I discovered that the reason units weren't responding to the addForce function was because they had already been killed (this was an issue with my mod). So the conclusion is: addForce doesn't work on units that have just been killed — it takes a few seconds before it starts working normally. During that time, even executing _man awake true has no effect. Sorry for wasting your time. I think this case can be closed.

So now only two issues remain:

1. Sometimes, after being blasted away by addForce, a unit may suddenly snap back to their original position.

2. Sometimes, after being blasted away by addForce, a unit seems to hit an invisible wall — all their velocity is instantly nullified, and they start falling straight down.