• Create new filter in basic mode.
• Set some conditions. Save. Only Save As is available.
• Change some conditions. Save becomes available.
• Switch to advanced mode. Save remains available.
• Modify filter and save without executing query.

Result is that the last modifications made in advanced mode are not saved.
In advanced mode, a filter must be executed before changes can be saved. So in switching from basic to advanced mode, the Save must be changed to Save As.