I haven’t personally seen this before. Some things to try here are:
Watering it down a little more so that it is less pasty and more liquidy.
If it’s still causing trouble, it sounds like they have a very slow digestion or low acid in the stomach. It is quite common to have acid reflux that is created from slow/poor digestion and low acid in the stomach. This causes the sphincter at the top of the stomach to lack tone and means that acid comes up the oesophagus from the stomach more easily. I would suggest trying some bitters with meals (only a few drops) and perhaps dandelion and milk thistle tea or capsules (if they do not have a risk of gallstones) and see how they are feeling trying this.
If it is a sensitivity to the marshmallow root you could try slippery elm (Ulmus rubra). The concern there is that there are sustainability concerns with this dear tree and it also costs a bit more than Marshmallow.