Tags not working?

That behavior is indeed strange, gittebanach. 

I replicated and was able to add a tag on the transactions in QBSE.  

Sometimes your browser becomes full of site data causing problems in QuickBooks Self-Employed. To check if this is a browser-related issue, you can open your QuickBooks using an incognito or private window. This can help us rule out any cache-related issue that might've caused the problems. Use these shortcut keys to open it: 

• Google Chrome: Press CTRL + Shift + N.
• Safari: Press Command + Shift + N.
• Mozilla Firefox: Press CTRL + Shift + P.

If it's now working well, you'll have to go back to your regular browser and clear its cache. This will delete the accumulated junk files and enhance your browsing experience.  

When you're done clearing the cache, close any browser instances to complete the process.  

Also, ensure you're using a supported and updated browser for QuickBooks Self-Employed.    

Additionally, you want to categorize your transactions each time you add one in QBSE. For the step-by-step guide, you can refer to this article: Categorize transactions in QuickBooks Self-Employed.

Feel free to drop a comment below if you have other concerns about tagging transactions in QBSE or other inquiries. I'm just around to help. Take care always.