Stephen CraverIt turns out the PICO's internal pull-up/pull-down resistors are somewhat undependable/unpredictable for anything but unconnected GPIO pins, as expained at https://forums.raspberrypi.com/viewtopic.php?f=144&t=315020
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...a perfect explanation! Thanks, Ken
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Possibly what that forum poster meant by "unconnected" is "configured the output to high impedance state" which is an option for pins allowing more that one output to share that line, as long as only one output drives it at any time. This is commonly called Hi-Z state.
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The statement that they are only good for unconnected outputs is meaningless. If the pin is unconnected then there is no need to pull anything down or up. A pull down or up is to ensure that any leakage current or residual voltage due to capacitance at the driven input doesn't affect the logic level when the output is "off". In other words don't let the input float. They should not be used to supply working current, the output transistor does that. So the pull resistance values inside the chip are moderately high.
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