Apparently, when exporting an InDesign file (.indd) to an InDesign markup file (.idml) there are some flaws, like incomplete text and misplaced text. As we know, the .idml file is the one used by Studio 2017 or 2019 for translation. These flaws are a known issue in the Adobe forums.
Hi Ozzie O
Thanks for your research! I don't know whether any of these problems is to blame for what you are experiencing, because the 8000th-character issue is extremely rare and also fixed by now and the other issues relate mostly to handling of graphics or aspects of text like kerning tags which should not affect the word count. (Hm, not 100% sure.)
What you can do _just to try it_: In InDesign Edit->InCopy->Export->All Stories, select a subdirectory to the location of the idml file, which you could name "Stories". InDesign will write lots of icml files into this folder, they contain the text which is now linked to the idml.
Create a folder "Stories backup" and copy all icml files into this. Don't touch unless you run into problems.
Drag these files into a Studio project (I recommend you start a "test" project if this is the first time.) If your InDesign is CC2019, you have to check "Unsupported versions" in the icml filetype.
Let Studio analyse the files and give you a word count.
After preparing for translation, translating, generating target files etc. Copy the icml files from the Studio target folder into the "Stories" folder. In InDesign's assignment tab you will now see that InDesign realized the story files were modified. Click on "update all". Voilá! Your target text is in the indd file. No idml conversion ever. Slick.