Once it is triggered no heating occurs for safety reasons. So cooling after it is to be expected.
The trick is to define timeout so that the decouple warning does not happen under normal conditions. Normally that means increasing decouple timeout. Without graphic of the curve I can not say much but looks like when you reach control range (normally +/-20°C) it is not heating enough to keep temperature and causes it. Try heat manager 0 for bed - it swings more then pid but is dead simple and the swings are not so important for print as extruder temperature.
