Did you add a spoiler or do something else that would increase the weight of your trunk lid? If so that could be causing your issue. There are two spring loaded bumpers on either side of the trunk lid latch that appear to lift the lid away from the latch just enough to keep it from re-latching the lid by itself. It takes very little hand pressure pushing down on the lid to compress those two bumpers to engage the latch, so adding weight to the trunk lid could compromise VW's design.
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I used my drivers door button to open the trunk to see if I could cause the same issue you have. I timed it after pushing the release button for 1 minute and it still did not go down by itself. Even after one minute I could lift it without any problem. Tried it again but did not lift and found that I could to push down slightly on the trunk until I heard a click and then the motor pulled it down tight. So, there does not seem to be any timer involved at least on my US 2007 EOS.
When I release my trunk it 'pops' then after a half second or so you hear a motor run for a second or two, I think that puts the latch in the 'up' position. My EOS has two spring loaded black rubber bumpers on the trunk lid on either side of the latch, these seem to be what pushes the lid up above the level where it will re-latch by itself. Pushing down compresses the bumpers and causes it to latch.
You might check to see if those black rubber bumpers move freely and that they are not stuck in the compressed position.
White 2007 EoS 2.0T DSG w 18" All seasons - love it