Yikes!

Breakers don't trip and fuses don't blow when the wire served is too resistive to provide enough fault current. The wire inside the extension cord is the fuse in this circuit. Although the conductor size is rated as a 15A circuit, it is very long which increase the voltage drop. This added resistance may limit the total current available to below the trip rating of the protector. Voltage drop increases as current and conductor temperature increases. This circuit description sounds like it is unable to supply the full voltage when continuously and substantially loaded.

Many NFPA rules are broken here. The input of the transformer must be supplied by a circuit capable of meeting the maximum input power rating of that transformer times 125%. It does look like it needs a 30A, 120VAC circuit.

Extension cords can never be used for continuous loads outdoors. You cannot bury the cord because it cannot dissipate enough heat underground. You cannot leave it in the sunlight because it is not rated and approved as UV resistant. It is also not "protected". All feeder circuits must be protected from damage, like within a wall, conduit or overhead, or buried.

You could just swing by Home Depot or Lowes, grab a 20A 2-pole breaker for your panel and 100' of 12 gauge, 3/12 w gnd NM-B direct burial cable and a 250V, 20A duplex receptacle and associated hardware to get a 240V circuit to your miners. That would be far easier than what you've done with that transformer.  That thing would sell on ebay well enough.