I have been caught a number of times, specifically when adding a new tag whilst entering a new task. The new tag defaults to OFF (unticked) so the task doesn't show. This seems counter intuitive. I can't think of a situation where I would want the default state to be hidden. The same applies to contexts, they too default to OFF.
When all the tick marks are on, it means "show only these tags that I have selected". When you create a new tag Toodledo makes no assumption that you want that tag selected. It is easier to understand if you have the tag filter with only 1 tag ticked. This means you want to see only this tag, so creating a new tag should not change the filter.
Try adjusting your tag and context filters so that all the tick marks are off. This will display everything unfiltered and newly added tags/contexts will be displayed without you having to adjust the filters.