It's been reproduced and demonstrated on enough algos and coins to conclude it's not isolated. And no one has demonstrated,
let alone claimed, it doesn't happen.

Anyway the first step in solving a problem is defining it. The definition of this problem is at a wall. Something happens to the
amount between the time a block is cleared an when it is credited to the balance.

The next step is to find out where and what, then why.

A simple control test can be performed mining a pool with one coin with one user and getting paid in the same coin.
Then extract all the DB info for it at various times, particularly between clearing and crediting a block. Is that too much to
ask?