mAh counts charge, watt-hours count energy

Milliamp-hours measure charge, not energy. To get energy you also need a voltage, and the voltage that matters here is the one inside the pack, not the one coming out of it. The standard figure for a lithium-ion cell is 3.7 volts nominal, and that is the number every conversion on this site uses.

The formula is short enough to do in a queue: watt-hours equal milliamp-hours divided by 1000, multiplied by 3.7. A 5000 mAh bank is about 18.5 Wh. A 10000 mAh bank is about 37 Wh. A 25000 mAh bank is about 92.5 Wh, which is under the 100 Wh threshold that carriers commonly publish, and close enough to it that it is worth knowing before you get to an airport rather than at one.

This is not an academic distinction. Airlines and aviation regulators state their thresholds in watt-hours precisely because milliamp-hours are meaningless without a voltage. A 25000 mAh bank at 3.7V and a 25000 mAh bank at some other cell arrangement are different amounts of energy. Watt-hours settle the question; milliamp-hours cannot.

The second half of the problem is that stored is not delivered. Your phone does not run on 3.7 volts from a bank; it is fed at five volts or higher, and stepping voltage up costs energy. Conversion losses are real, heat is real, and if the bank transfers wirelessly through a magnetic coupling the losses are larger again. This is why a 10000 mAh bank does not charge a 5000 mAh phone twice, and why nobody should feel cheated when it does not.

One more thing worth saying plainly, because it is a safety matter rather than an arithmetic one: a battery that is swollen, punctured, wet, or hot to the touch should not be charged and should not be used. Do not open it and do not try to repair it. Follow local guidance for battery disposal.