No it's not a Custom Visualization, it's a bar graph. Yes, I understand we can't pin slicers. One things to add, the page which contains the bar graph also contains a custom visualization. Not sure if the custom visulization will impact that.
Same thing happened to me today... Imported data from Excel into Power BI Desktop and published report from there. Going onto power Bi online and I am unable to pin any visualizations with the orgional report. I can pin the whole report, make a new report from the same dataset and pin those visualizations, or duplicate the report and pin from there. No idea what is causing it for me... I have tried creating new datasets and those have the same issue as well when published from power Bi desktop, where as old reports work just fine.