It's a bit unintuitive, but wealth rewards are scaled down by the level of wealth you reach. Once you reach Solvent wealth, rewards of 3 or less do not increase your wealth anymore.
Rerolls then (are intended to) set you to the max wealth you would have reached if you got all of your rewards up to that checkpoint in a single run. Currently you should cap out at Solvent 1/10 or maybe 2/10.
The situation you describe is a gap/optimization you could try to take advantage of during a single run, where you spend down below a Wealth tier before completing a mission so that you receive an extra "unscaled reward."
This can give you slightly more buying power for that run, but the benefit of that is pretty limited; basically you can buy an extra tier 1 item or two, at a time when you should already be able to fill most of your active party's inventory with them.
Typing it all out, it definitely feels confusing, and I get how it feels like you "lost wealth" unexpectedly. I'll consider if there's a way to make it clearer; perhaps closing that single run gap is the best option, or maybe there's a better way.