Device: Samsung Galaxy Note 10 (Android 10), Google Pixel 3 XL (Android 9).
Build: Fenix Nightly/Firefox Preview Nightly 5/26.

Steps to reproduce:

1. Go to https://dataride.uci.ch/Results/iframe/EventResults/220062?competitionId=59800&disciplineId=7
2. Long tap on the first cell from the table and select a couple of rows.
3. Go to https://results.totallympics.com/forum.php?mod=post&action=newthread&fid=71
4. Open Forum and start a new Thread.
5. Go to the right of the edit area and Uncheck the Code checkbox.
6. Paste the information from the clipboard.

Expected Result: The entire format of the table is present.

Actual Result: The in-between cells and margin format is not present (header of the table).

I pasted the copied data to https://html-online.com/editor/, I didn't see the same issue.
Not sure if it is a website issue. I don't have account on totallympics.com, could you help to test how does Chrome behave? Thanks.

Reproducible on Ubuntu 18.04 too, works with Chrome.

Reproducible on Firefox Preview Beta 5.2.0-beta.1 GV:78 with Google Pixel 3 XL (Android 9) and Samsung Galaxy S9 (Android 8).

And a small remark is that when selecting 2 rows from https://dataride.uci.ch/Results/iframe/EventResults/220062?competitionId=59800&disciplineId=7 , the last selected cell from the last column is not highlighted, even though when it is pasted it's displayed ok.

Apart from dropping the table's style bug 1646515 this is a website-specific problem as the pasted markup contains the <th>s.