I don’t think this is supposed to happen; it doesn’t happen when creating new pedal lines from the popover or sidebar, only when copy and pasting. (Which is not usually a preferred option, but sometimes useful.)
Have also replicated the problem when copy and pasting una corda lines over a sustain pedal line.
I can’t get the video to load on my browser.
That is weird. See if this works:
Yes you new video does display (w/o sound, which is okay).
Just to clarify, could you describe the problem you wish us to notice?
@naomi This looks funny, indeed, pedal markings eating up each other! 
Seems to be a bug, just tried it out myself and can confirm you’re right: If you copy small pedal movements over larger other pedal holds, the holds will become shorter with each copy.
Pedaling Problems.dorico (491.8 KB)
Thanks for reporting this. I can confirm that this isn’t working exactly as we would expect. Dorico does try to truncate existing pedal lines if you create overlapping lines by pasting, but this is happening slightly overzealously when the range of the new line is completely contained within the range of an existing line; it shouldn’t really change the duration of the longer line in this case. I’ve made a note of this and we’ll take a look at fixing it in future.
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