It’s strange.
You can Copy/PasteSpecial a single formula of that kind from a spreadsheet (Calc) to a text document (Writer) using Paste Special...
in RTF mode. (This changes the relative size of sub/super, however, a bit.) If you try to import formulae from a CellRange this way, RTF will create a table and ignore Sub/Super.
RTF is an old mess, I can’t help. I would suppose LibreOffice tries to reproduce a behaviour originally introduced by MS. And everything going through the clipboard is afflicted by the properties of that tool and the formats it supports (OS dependent).
Anyway “formulae” based on character styles are very limited. You cannot have a superscript (high index) above a subscript (low index) e.g. as is needed for the common nuclide notation or for a chemical application where charge signs should display above counting indices or the like.
To avoid problems of the kind you may try the ‘Math’ component of LibreOffice which can be used to create formulae as OLE objects in Writer and in Calc as well. No difference in appearance then.