The expense by vendor report has not specified on it I use to be able to go to filter and pick specify to clear it out. I can't anymore why
I tried to run the Expense by Vendor Summary report to check and I can still see the Not Specified option in the Filter section, betsym5255.
A possible cache or browser issue can be a possible reason why you can't see it when customizing the report. I'll some steps with you on how you can resolve common browser issue when working in QuickBooks Online.
You can start by using an incognito/private browsing session just in case the same issue appears. Use these keyboard shortcuts to initiate an incognito browsing session:
Ctrl + Shift + N for Google Chrome
Ctrl + Shift + P for Firefox
Control + Option +P if you're using Safari
Ctrl + Shift + P for Microsoft Edge
You can also delete cache or use any of browsers that I've enumerated above. They all work well with QBO.
Once done, go back to the report and check if you can already see the Not Specified filter.
Feel free to check out these articles as well in case you might need them when running reports in the future: