Merge sequences warning is shown only first time

1) Click "Open file" button.

2) Choose two sequence files with different alphabets (e.g. "_common_data/fasta/DNA.fa" and "_common_data/fasta/amino_multy.fa") and click "Open" button.
Expected state: the dialog appears.

3) Choose "Merge sequence mode" and click "OK" button.
Expected state: the warning about different alphabets appears.

4) Close the project and repeat these steps.
Current state: the warning does not appear.