If a cell (or group of cells) contains the @ (at) character, doing a Find & Replace action will replace the @ character with the Replace string, instead of the character indicated in the Find field.

How to replicate: 

1) Select a cell containing an email address
2) Type any string on the Find field (e.g. a period)
3) Type any string on the Replace field (or leave blank)

Pressing Replace will substitute the @ character with the string in the Replace field or simply delete the @ character if the Replace field was blank

NOTE: this doesn't happen on today's MinGW master build or in LibO dev 3.5 (Build ID: 8cc839d-3dca5fd-4525197)