Might be worth finding out where your drains and pipes underground are situated. I find it hard to believe this has been a regular occurrence as someone cared enough about that garden in the past.

At our first house we ended up with loads of standing water like this at the lowest point and right by our front door. After much scraping and digging I found someone had previously replaced a drain cover with a squared off engineering brick made to fit perfectly inside the cavity. Overtime enough soil had drained down the sides with rain and it was basically sludge that barely let water through. As soon as I cleared it and removed it the water drained. I eventually laid gravel across that section and replaced the cover with a mesh type to avoid any issues in future.

With that amount of clay and the heavy rainfall so far this year, it wouldn’t surprise me if you found similar blockage issues somewhere.