While using OpenShot 2.4.3 the timeline window shows small blue tick marks that appear at the bottom of the timeline as I move the playhead. All else seems to work well.

That is not a bug... that is showing how much of the preview has been rendered and cached.
As @N3WWN says, that's the preview cache status display. (Which I _believe_ is documented in the manual, though I'll double-check before closing this.)
Huh, I guess it isn't! OK, a documentation bug, then. I'll add it to the list.
Thank you so much for submitting an issue to help improve OpenShot Video Editor. We are sorry about this, but this particular issue has gone unnoticed for quite some time. To help keep the OpenShot GitHub Issue Tracker organized and focused, we must ensure that every issue is correctly labelled and triaged, to get the proper attention.
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Huh, I guess it isn't! OK, a documentation bug, then. I'll add it to the list.