gtk_progress_bar_set_fraction with certain values causes text to disappear

I have the following layout:

 GtkWidget *progress = gtk_progress_bar_new();
 gtk_progress_bar_set_show_text(GTK_PROGRESS_BAR(progress), TRUE);

 GtkWidget *elapsedTime = gtk_label_new("00:00:00.0");
 GtkWidget *totalTime = gtk_label_new("00:00:00.0");

 GtkWidget *hlayout = gtk_hbox_new(FALSE, 4);
 gtk_box_pack_start(GTK_BOX(hlayout), elapsedTime, FALSE, FALSE, 4);
 gtk_box_pack_start(GTK_BOX(hlayout), progress, TRUE, TRUE, 0);
 gtk_box_pack_start(GTK_BOX(hlayout), totalTime, FALSE, FALSE, 4);

 GtkWidget *layout = gtk_vbox_new(TRUE, 0);
 gtk_box_pack_start(GTK_BOX(layout), hlayout, FALSE, FALSE, 0);

That I update periodically:

 gtk_progress_bar_set_text(GTK_PROGRESS_BAR(progress), "Text");
 gtk_progress_bar_set_fraction(GTK_PROGRESS_BAR(progress), fraction);

 gtk_label_set_text(GTK_LABEL(elapsedTime), "00:00");
 gtk_label_set_text(GTK_LABEL(totalTime), "00:00");

If fraction is set to certain values:

 double fraction = 0.003033; // bad
 double fraction = 0.004; // bad

the text in progress and totalTime fails to display in the Ambiance and Radiance themes, while it does display in the Adwaita and High Contrast themes.

Other values:

 double fraction = 0.003; // good
 double fraction = 0.005; // good

the text renders without a problem in all themes.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: light-themes 0.1.8.25
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic 3.0.4
Uname: Linux 3.0.0-12-generic x86_64
ApportVersion: 1.23-0ubuntu3
Architecture: amd64
CheckboxSubmission: 5434425585a31b50d9fa7c3153a37fe1
CheckboxSystem: b845c366ea09c60efa3a45c1b5b21525
Date: Fri Nov 11 07:52:40 2011
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100318)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: light-themes
UpgradeStatus: Upgraded to oneiric on 2011-09-24 (47 days ago)