I wouldn't worry so much about acid core solder as much
as lead content. Was it lead-free or the older type?

HTH
Chris

Acid flux eats copper. Acid flux is needed for steel.
In years gone by, I would say change the solder, wipe the joint, sand the joint to remove the acid, then redo the joint. In todays life, the heater probably won't last long enough to make a difference. You can tell when someone used the wrong flux. After a period of time, the joint is white and corroded. Sometimes this takes a week, and sometimes it takes a year.

Unless you installed a very long life heater (more than 6-7 years) forget about it. If you used flex lines to couple the heater, they will probably leak first.

I wouldn't worry about it at all. The acid only has an effect until it's neutralized. Many homes have acidic water, including mine, which has a cumulative effect on copper. It took 20 years to eat through the L copper pipes