I don't use UTAU Synth, but that looks very similar to when you import a MIDI on Windows UTAU. I know that's not a MIDI given the title of the project, but.
If you scroll all the way to the very start of the UST, is there a tempo marker anywhere on the left hand side? (In this case, the 500000.00 under measure 0.)
Here's what I'm talking about, with Windows UTAU for reference. This may not be a thing UTAU Synth does at all, but.
View attachment 8779
If you see something like that, maybe you can right click and delete it? You can do that on Win UTAU.
Normally you'd have to reset the tempo too (note my tempo box), but it looks like the tempo of your project is fine.
Like I said, this may not apply to UTAU Synth at all, so sorry if this isn't any help.
@Kiyoteru (sorry to tag you!!) uses UTAU Synth. Maybe Kiyoteru can be of help in a way I can't be.