After doing extensive testing, I believe all of the reported issues could be explained by having an annotation filter that might not show in your photo browser options (except in some cases under Grouping - see my other reply) and contains a value that is not relevant to the current taxon. The end result is that nothing matches.
For the impatient, here’s a workaround:
Skip all my lengthy explanation and just find a taxon that has an annotation filter that is set (i.e. doesn’t have “Any” value). If you’re having trouble in one particular taxonomic tree, go to a different one (like if it is Flowering plants that don’t show up, try Animals, and vice versa). The key is to find one that has a menu from which you can choose “Any” again to clear the stuck filter. Once you do that, you should be able to see all photos in your other taxa again.
What’s going on and how to reproduce it:
The last time I was in photo browser I clearly recall I had filtered on life stage = adult. Today, I tried the photo browser for a taxon for which life stage is entirely not applicable (Usnea longissima). It shows no photos, and no option to unset life stage:
Behind the scenes, it is trying to apply the life stage filter via this API call: https://api.inaturalist.org/v1/observations?verifiable=true&taxon_id=122681&place_id=&preferred_place_id=6853&locale=en&order_by=votes&quality_grade=any&term_id=1&term_value_id=2&page=&per_page=12
Note the presence of &term_id=1&term_value_id=2 filter in the query. That should not be there.
- Toggling grouping was ineffective.
- Navigating to Arthropoda and opening the photo browser gave me access to life stage once more.
- From here, I was able to clear the life stage filter.
- Then, I navigated back to the problematic display: https://www.inaturalist.org/taxa/122681-Usnea-longissima
- The photo browser shows photos again:
And of course, the observations API call URL no longer includes the annotation filter that was screwing up the search: https://api.inaturalist.org/v1/observations?verifiable=true&taxon_id=122681&place_id=&preferred_place_id=6853&locale=en&order_by=votes&quality_grade=any&page=&per_page=12