I suspected it was something like that. I’ve never used tables in Scrivener, I don’t think. But what you described brought to mind something I’ve seen with lists in Scrivener. And pasting between Scrivener and Google Sheets, and Docs and Gmail too, has formatting issues, maybe even more than between Scrivener and Word, which is somewhat improved since Scriv Win 3.
I believe I’ve confirmed what you’ve found.
Just now, I pasted a small range of cells directly from Excel into Scrivener, and it looked as expected, including cell borders and some bold formatting (all else default in Excel.)
I then pasted those same cells from Excel into a new google sheet, and it too looked as expected.
Then, I copied again the cells I just pasted into the google sheet, and pasted them into a new Scriv doc, and they pasted, including some bold text, but without visible cell borders.
Then I closed the Scrivener project, and then reopened it, and the tables were broken, as expected and as you described, just a jumble of misproportioned cells and squeezed text. The one from Excel still had gridlines, the one from google sheets still did not.
I didn’t check to see if the tables were adjustable, but they’d still need to survive being closed and reopened, again and again. You’d also want to make sure they could survive even a window resize. Lists in Scrivener are fragile, too. I’ve learned to avoid them, even within Scriv, without cross pasting.