While preparing a test image to properly show the issue, I accidentally found out what causes the behaviour: it's the Timeline proxy mode setting. When turned off everything behaves like expected. But once it's set to 1/2 or 1/4 the issue arises. Additionally interesting: It does not happen on "standard" 16:9 Timelines. I'm working with two 4:3 Timelines (1440*1080) and it looks like this:
(The "2Channel preview Timeline" visible in the screengrabs is 2880*1440)
Proxy mode setting: off
Proxy mode setting: 1/2
(Clips stay centered of course, the offset is only due to my skewed test image)
Setting proxy mode to 1/4 prompts a further zoom in.
Since I'm running low on hardware power I'm usually working at least in the 1/2 setting, so would be great to have a solution for this. Otherwise I'd have to reduce the size/bitrate of my proxy files to a point where I can flawlessly edit with proxy mode off.