I don't have an answer as to why. However, I would be curious to know what happens if you put a Volt Meter a crossed the Battery Terminals when you attempt to start when it is Hot.

If it clicks when and the Voltage remains on the High side that would be an indication that the Solenoid goes back the plate inside is not making contact with the 2 Contacts that send the Voltage to the Starter Brushes.

Do the same test when everything is cold and see what the difference is.

Also Your Starter designed similar to a Delco Starter from the 1960s and 1970s so if you found a troubleshooting Guide for that it might apply to Your Starter

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84 300D, 82 Volvo 244Gl Diesel