bookmarks missing after copying document

After a Windows 10 update my main ODT document that I use daily was not working correctly – very slow and often hourglass or “not responding.” Other documents seem to have the same problem if the main doc is open but work OK if it is closed. The same document on my second PC is working OK. I tried deleting some Win 10 updates but it did not help. I tried saving a copy of the working doc on PC2 to PC1 and the same problem continued. I determined via Safe Mode that the problem was not with the User Profile. Finally I put all the text in the doc on PC2 onto my clipboard and pasted into a new doc on PC1. This appeared to work BUT many bookmarks are missing.

Doc is 442 normal pages, 104K words. 8.55KB “size”, 12KB “Size on Disk”. Many bookmarks and frames. Using Writer.
On the original doc I have the page height set to 118" to minimize page breaks (I never need to print the document). I set the page to 11" when doing the copy in case that was creating a problem but this made it so that some of the framed information was lost and didn’t seem to solve the problem anyway so I went back to 118" in a new receiving document but the bookmarks were still missing. It will be pretty tedious to have to recreate the many missing bookmarks although I’m relieved to have a working document with no text missing. Any ideas of how I could perhaps export/import the bookmarks as a separate file or somehow restore what’s missing? TIA

LO (X64) Win10 Home V 1903 Intel Core i7-4770