Am I the only one not seeing a problem here?! This just looks like a simple text formatting related thing to me.
The lines reading "a1" and "asd12125" are in the same font and are set to bold. The line containing just the numbers are NOT bold and appear to be in a completely different font.
Either select all three lines - then change the font so they are all the same font and then use the bold tool to set the boldness property appropriately for all of the selected lines.
Or just select the middle line and set its properties to match the other two lines. Either of those options should solve the problem surely?!
If neither of those options fixes the problem for that middle line of text - then perhaps try selecting the middle line and then either press ctrl+M, or select the menu item "Format->Clear Direct Formatting" to remove any special formatting from that line and reset it to the default.
Or select ALL of the text and use ctrl+M, or the afore-mentioned menu item to remove/reset the formatting for the entire document.
And if that fails to work - the only remaining possibility is that you, or someone else has changed something in the default style that affects the formatting of that line somehow. In which case - perhaps you need to reset your preferences/settings for libreoffice. To reset your user-profile - see:
wiki.documentfoundation.org