Illustrator 30: text has corrupted and retro-activley corrupted timemachine versions

Hi all.
This is not easy to describe but if it sounds familiar i'd be interested to hear of a fix.
A drawing i've been working on has developed some sort of corruption.

It comprises 5 artboards with some relatively simple architectural drawings, text objects, text boxes and some pattern fills.

I exported the pages to PDF. Then reopened the file to make a change.

`i discovered all the text objects in the file had jumped off their artboards to one side.
They had maintained their relative positions. 
I would normally think that somehow I had moved the text accidently myself. Except that where text was in a box, the text had moved but its box remained in its original place.
When I opened timemachine versions of the file, the fault was imposed onto those versions as well.

I have reinstalled illustrator but the problem persists.

I have had to split the drawing as separate files, 1 artboard per file and am pasting in type from the PDF export of this file.
I would like to know what, if anything caused this as i lost a day of work looking at this and am still repairing the damage. Am also worried the problem will randomly repeat itself.

Thanks in advance