OP has created a duplicate topic in Steam's PoE forum. This is a waste of community resources.
The Steam app is smart enough to cache unlocked achievements even when playing offline, and it syncs them to the Steam account when getting online again.
There have been a very few cases in 2015 and 2016 in Steam's forum where players had to verify the installation files to unlock at least one achievement. In at least one case they've simply restarted the Steam app. But generally, in Steam's forum I don't remember any common problems with unlocking achievements. Quite the contrary. There have been multiple players, who've unlocked all achievements since 2016.
Your logic is completely laughable. You're saying that because there is no known issue and that "Steam is smart enough" this is a non-issue.
However, I'm telling you, Belf, that is definitely a problem for me. And I'm simply looking for help where I can find it.
The only resource here is time. And everyone on this board has plenty of it, I assure you (that's why we have time to play games like PoE).