When overwriting a blueprint by giving a new blueprint the same name, the old icon is discarded in favor of whatever was entered in the Save Blueprint window. The preferable behavior would be to keep the old icon so that you don't have to recreate it from scratch every time. It should probably still discard the old icon if the icon in the Save Blueprint window was modified in any way, in case the player wanted to customize it right there, but if the icon wasn't edited away from the default and the overwritten blueprint has a customized icon it's pretty unambiguous that the desired outcome would be keeping the old icon.
Tobias told me to report the first part of #1262438368046874634 message as a bug, so here it is.