If styles are changing it's likely that you've based at least some of them on [Basic Paragraph] and that definition was edited in the old file. This is the main reason most of the experienced users here recommend against ever using or basing anything on [Basic Paragraph].
You should be able to fix any style definition problems by clicking the flyout menu in either the Paragraph Styles or Character Styles panel and choosing Load All Text Styles..., choose the old doc as the source, and check the box to use incoming styles in case of conflict.
You will be far better off working with a new doc than one that has already shown corruption.
Thanks for this solution - I had a similar problem (2025 - InDD 19.5.2.x64 - can't upgrade as I have an older PC) and struggled until I found this thread. Luckily I keep all versions until the end of the project so could reimport char styles.from an original file. Then I changed the style to a completely different one for the section that I had managed to isolate as the problem.
The problem seemed to come from a Word file where Footnotes had been converted to Endnotes and back again. I would suggest if you do have to import a file like that and it is short enough, just get it into a text file (I printed to PDF and exported as a text file from there) to strip out as much formatting as possible. it is a pain to put the formatting back in but I couldn't find another way around it.