OP: are you referring to the EIN of the entity whose return you are filing (input on page1/Client Info)?
  Or--as some respondents evidently think--the EIN of some party who is paying you (1099 issuer)?

If it is the second one, I agree: there may be an issue with your tables of saved payer information.  This is why I dislike using tables of saved information, and long-hand type all of my entries.

If it is the first one, and your CLIENT info is getting changed, there may be "something goofy" with your primary data table.
 - Try CLPack (I believe this is now in the Lacerte Tool Hub).
 - Leave that client there, Ctrl+N(change Name) it to "#####BAD" (whatever the old number was, and suffix it somehow), and then Ctrl+A(dd) the actual client number again.  This preserves that data record in the client info table (in case there was something strange with just that one line of data), and forces a new line entirely.
 - Copy/Paste the EIN from a clean spot (like a blank/new NotePad file you opened).



Robert