Thanks for the responses.

For me, the problem seems to be in the stomach so, instead of vinegar, I tried Betaine HCL and it seems to be helping. I've also ordered some proteolytic enzymes (protease) that break down protein.

The following 2021 paper says protein digestion in the elderly is a common problem and they suggest using aged meat, enzymes and cooking below 100degC, ideally sous vide:
https://sci-hub.se/10.1016/j.foodchem.2021.129707
(If you're outside the US, you may need to use a VPN to access)