Hello!
Great to hear that a fix is coming soon. 
I wanted to have a table that shows the click rate (clicks / sessions) on specific custom events, based on their category. However, as it is not possible at the moment to prefilter a custom metric in Piwik PRO (I don’t know if it’s in your roadmap, but it would be nice ), I chose to create this table via Looker Studio. For this, I created a blend with:
- For the first table: all sessions
- For the second table: custom events with a filter to only show the ones with the desired category
It worked well, and for whatever reason, I tried to add filters using this blend too. That’s when I saw it worked!
EDIT
I just saw this post asking about filtered calculated metrics : Using a filter in a Calculated Metrics - #2 by Jarek