When using the completion proposals on typing (code completion/ content assist):

the proposals are still invoked when a statement terminator (like ;) has been entered.
When pressing enter, the proposal is used an not a new line has been added. See this cdt-lsp issue: eclipse-cdt/cdt-lsp#550

The proposal is valid since in C++ its allowed to start the next statement after a space (see clangd/clangd#2508 (comment))

IMO in 90% the user wants to enter a new line after a ;