Created attachment 85725 [details]
One of the font files used

There are 5 files included:
2 font files Saumil_guj2 and gujfonts folder
2 PNG files with screen shots
1 open office extension file for 'gx button'

Open Office 4.1.2 (office) on Windows 10 64x

Unknowns:
What text file the original post used for Gujarati, therefore I don't know if my text file (tested 2 different font files for the same language) matches that of the original bug. The text files I used are attached.

What the language settings were so I tried two combinations of:
English, Gujarati
English, no Gujarati
To find languages, Tools->Language->('all text' was used of options available)->More

In the office version of the original post, there was an extension 'gxliterate' that has unknown compatibility with office 4 and above as of now. I didn't need to download this extension (although attempted before downloading text files and font wasn't an option) however research leads me to believe that the extension generates a button on the tool bar labeled 'gx' which wasn't appearing on my version. I'm not sure if this was used on the original post but it seemed to be the way to have the font in previous versions appear/translate while typing between languages. File is available for download, search Gujarati on the available extensions online. 

Reproduction:
I opened and reloaded the open office writer document 20-25 times total (about 10 each combination) and never had missing text. For every test, the same exact text/files were used and I had 'The dog is black' followed by layers of the same phrase; both written with the Gujarati font previously selected and converting English characters to Gujarati (using both text files which I named differently to be able to install). 

Further investigation:
Test with different colors on the characters or font sizes