I am going to offer up a MacroDroid solution to killing MLB.
You just said that the WiFi timer turns off MLB, but it is staying "on screen" but the tablet is NOT turning 'OFF', am I correct?
Try this sequence and let me know if it works for you.
Trigger: WiFi disabled
Action: Kill Background App
Select: MLB
Action:
Select: Screen OFF
NOTE: when you select Screen OFF, MacroDroid is going to ask you to allow it to be a Device Administrator. You must answer 'yes' or it won't work.
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You should be able to test this quickly by manually turning off WiFi and watching to see what happens next, or just moving out of range of the WiFi and see what happens.