IME, such plug/outlet heating is normal when the appliance draws near
the rated current of the circuit for "long" periods of time. Plug-in
electrical heaters are a standard culprit.

In one work situation, local Fire Marshal would go ballistic on finding
any plug-in electric heaters. The sort of unit many secretaries MUST
have. Mainly because of the insidious way they fail. Day after day,
they work fine. Then POOF!

Of course, mfg. cost-cutting doesn't help, either with plugs or cables
or outlets or connectors. For any sustained heavy load, you want the
best available, even if it costs $.50 more.

J