Mogrt's placeholder timecode bears no resemblance to anything

I've created a mogrt in AE which I've dropped into my sequence. It has a video placeholder.

When I drop some footage into the placeholder thing in the Properties panel, I assume I could then set the clip's in-point using the "In Point" setting in the Properties Panel. But no - this seems to bear no relation to the clip's timecode.

The mogrt is an overlay for an interview; a text question appears on screen as the footage "behind" it (the footage that replaces the placeholder in the mogrt) is blurred and de-contrasted. So I want the timing of the placeholder clip to be exactly aligned with the existing footage in my sequence. The only way to accurately do this is to trim a portion of the clip in the sequence, cut it, and paste it into the "Background" in the timeline that's created when I open the hamburger menu in the mogrt's Properties panel and do "Open in Timeline". In there, I can line it up between the hatched areas nicely. Back in the main sequence, I see that the resulting in point timecode (00:01:34:10) bears no relation to the actual in-point of the clip (05:13:40:15). What is going on here? Everything (the source footage, the sequence, the mogrt, the placeholder 4K video) is 25fps 4K.