The default value for showSmallPagingText is TRUE (noted on the Config Options page).
If, when you first open JuiceboxBuilder-Pro, you do not change a configuration option, JuiceboxBuilder-Pro will not write the option's default value to the gallery's 'config.xml' file (to save cluttering up the file with hundreds of entries) and Juicebox-Pro will use the default value when the gallery is displayed.
If there is no entry for showSmallPagingText in the gallery's 'config.xml' file, then Juicebox will use this option's default value of TRUE and you'll see the thumbnail paging text when the gallery is displayed in Small Screen Mode.
I think this might be what you are experiencing.
I have checked that JuiceboxBuilder-Pro is writing correct values to the 'config.xml' file and cannot find an error.
Correct values (corresponding to the interface input fields) are written to the file (but default values are not always entered).
I hope this helps somewhat.
If you have a gallery which does not behave as expected, then please post a link so that I can investigate further.
Likewise, if you have a set of steps that I can use to reproduce an error with JuiceboxBuilder-Pro, please let me know.
To summarize...
showPagingText="TRUE" will always show the thumbnail paging text when the gallery is displayed in Large Screen Mode. The default value for showPagingText is FALSE.
showSmallPagingText="TRUE" will always show the thumbnail paging text when the gallery is displayed in Small Screen Mode. The default value for showSmallPagingText is TRUE.
If a configuration option is not explicitly written to a gallery's 'config.xml' file, then Juicebox will use the default value for that option.
Also, just to clarify, you mention "LargeScreen Mode in mobile browser" and "LargeScreen Mode on mobile devices with screens smaller than 1024px". The only way that Juicebox will be displayed in Large Screen Mode on mobile devices with small screens is if you set screenMode="LARGE". If the gallery is truly in Large Screen Mode, then showPagingText will be active (and showSmallPagingText will be ignored), regardless of the device or browser window size.