For a while I've been aware that trying to do achievements when not being online doesn't work. Therefore I've resorted to checking whether I've got access seeing my current achievements before I do a new achievement. This has mostly worked nice.

Today, this failed however, and I lost both getting the Cure a Zombie villager achievement, and progress of a few hundred emeralds on the Master Trader achievement. 

What I did was the following:

• Restarted the game, and made sure I was logged in on my Microsoft account
• After starting the game I prepared for curing a villager, and as the final step checked once again that I could see my Achievements (indicating that network was fine at this stage)
• Cured the zombie villager, and started trading with another villager at discount prices. 
• Did not receive notification that I got the achievement, which in hindsight should have triggered an alert related to network connectivity (or loss thereof). Not however, this only affects the Nintendo Switch/Minecraft. The network in my home was online throughout this session
• Kept on trading, and after an hour or so, tried checking my achievements and it got stuck with "Fetching achievements" and nothing happened 

So the bug (/missing feature) as I see it is two-folded:

• There should be an in-game storage of recently (local) acquired achievements, which should synchronise when the game's network issues has been resolved
• If achievements are temporarily disabled due to some network issue in the game, a notification should alert the user of the current network issue

I've also experienced similar issues (related to other achievements) when playing on both iOS and Android, so this is not limited to the Nintendo Switch, but is a general bug related to acquiring achievements with temporal network issues and the missing local storage of the acquired achievements.