Can't enter slurs on first note of bar

Very strange issue encountered with one specific project. All of the music in this project was imported via MusicXML; however, I haven’t noticed the same issue in any other projects, including other projects containing MusicXML imports.

When in Note Input mode I cannot enter slurs on the first note of any bar. Hitting S before or after entering the note shows that a slur is active on the left sidebar, but it does not appear until the third note of the bar, and the first note is not contained under the slur.

I’ve found that this behaviour is replicated if I a) split a flow from the affected project into a new file; b) copy and paste all of the music into a new file; c) delete everything except the first page. This issue is not present if I try to manually re-enter the music into a new project, but this is a long project and that would be very time-consuming.

I’m attaching the first page of the project for space saving reasons, and since it displays the issue by itself, at least on my system. Dorico 4.
piano quartet - I.dorico (2.2 MB)

When entering notes via the QWERTY Keyboard into an empty measure, I can reproduce this problem even though my copy of Dorico converts the file to v. 4.1 and even after I applied Reset Appearance to the file.

(Tried to load a Diagnostic file, but it was too big. I emailed it to Daniel.)

I reproduce this in 4.0, and not in 3.5. Also, if I delete the notes I entered and try again, the slur-delay problem does not occur. Mysterious.