Loading a custom board sets the board size, number of mines, and max mines per cell. Of those, only the board size should be permanent - when a new game is started the number of mines and max per cell should be reverted. Currently the max mines per cell are accidentally reverted.
This gets messy and feels a bit of a weird behaviour anyway (although per cell will continue not to be changed in response to loading a board). Consider the following:
Play a custom board of beginner size with custom mines
Load a beginner size board (possibly an actual beginner board)
Should a new game revert to the custom board number of mines, stay the same as the loaded board, or revert to the standard number of mines for that size board?
Loading a custom board sets the board size, number of mines, and max mines per cell. Of those, only the board size should be permanent - when a new game is started the number of mines and max per cell should be reverted. Currently the max mines per cell are accidentally reverted.