Is it just at that fixture? For instance as a test, can you run that until it starts to drop temp and then go try the bathroom sink faucet to see it its also cooler? Sometimes the fault can be within the temperature balancing mechanism of the shower cartridge. Of course the heater will be the culprit if that faucet is also not as hot as you're used to.
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"The trouble with people idiot-proofing things, is the resulting evolution of the idiot." Me
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