This may be related.
I was pasting a lot of text in my bbPress import thread. I was pasting it in as code. Here is what was happening: I was in the post editor. I'd click on the Code button to insert code. I'd paste in my code, then click Insert. When I went back to the post and hit return, it would delete some (but not all) of what I had just posted.
I'm testing with this post, so hang on a second...
PHP:
$ImpExData_object = new ImpExData($Db_target, $sessionobject, 'attachment');
OK, here's how I duplicated it. If I just
type code into the popup code editor and insert it, it works fine. If I
paste my code into the code editor that pops up, this is when things get interesting. On the PHP code I just pasted in above, the blinking cursor was gone. I clicked after the closing [/PHP] tag, got the cursor back, hit <enter> on the keyboard, and the closing [/PHP] BBCode got deleted. Ctrl-Z (undo) brought it back.
BTW, the first character in what I posted is a tab, which is what indents it.
Trying another...hang on...
PHP:
$ImpExData_object = new ImpExData($Db_target, $sessionobject, 'attachment');
foreach ($data_array['data'] as $import_id => $data)
{
$try = (phpversion() < '5' ? $ImpExData_object : clone($ImpExData_object));
Yep, repeated it. I pasted all of this in from my editor (Homesite+ 5.5), so it should be plaintext. This time, the cursor was underneath the closing [/PHP] code, and when I hit <enter>, everything except for the first line was deleted. Again, Ctrl-Z brought it back. Apparently, whatever is happening, it leaves the first line untouched but deletes all others below it when I hit <enter>.
So, not sure if this is a TinyMCE issue or not. Most likely it is, but at least I've found a way to duplicate in this instance.