reading the tea leaves, it probably came down to Martin vs Miranda. Martin is clearly a better fielder, but Miranda is hitting better at this point. I believe both players still have options.....
following further research - Martin has all 3 option years available. Miranda is a little tricky. He has one option year left - but if he does not spend 20 days in the minors this season, that does not count as using an option year, so he would retain the option for next year. He only played 7 games at AAA, so as far as I know, he has not used his option for 2024. The Twins may be thinking that, if they keep Miranda on the major-league roster all season, then they retain his option for 2025.
If a player does not use an option year, it carries over. so if a player is added to the 40-man roster but never appears in the minors, that player retains all 3 option years. however, once you reach a certain point, you can refuse to be optioned to the minors (except on a rehab assignment).