Could human stomach acid be bioengineered to break down microplastics?

I don't know, but I don't think hydrochloric acid would be able to break down plastics. However, maybe we could find enzymes that would be able to.
I found this article that says we already have but they don't work at ambient temperature : https://news.utexas.edu/2022/04/27/plastic-eating-enzyme-could-eliminate-billions-of-tons-of-landfill-waste/
Also this one that talks about the subject : https://www.sciencedirect.com/science/article/pii/S0167779921000408

I'm fairly sure you would not want to break them down so your body could absorb whatever is in them more easily.

You would need a solvent that would break down organic molecules that create the strength for plastic. Acid isn't good at breaking that down.
Any material that could break down plastic would also break down everything that makes food useful for a body.

Surely we would not have our bodies so full of microplastic if our stomachs could destroy them.

I think the problem is that any acid strong enough to break down the bonds of plastic would be too strong to comfortably sit in our stomachs.
Maybe you could add some of those plastic eating bacteria with like an artificial fecal transplant? Not sure what the consequences of introducing those to a gut biome would be.
- Yeah, why do stomach when you could add it to the biliary enzymes

Add plastic eating bacteria to our gut biomes maybe?
- You could make them capable of eating plastic, but what would the byproducts be? You would need to make sure the byproducts aren’t toxic.
  
  Oh, just harmless acetone