Where in the world are you? That USDA Plant Hardiness Zone only tells us which plant might survive an average winter in your garden.
If water is still standing in that hole 24 hours after filling you do have a drainage problem. Drainage problems have numerous causes but most common with clay is not adequate levels of organic matter that will separate the clay particles and allow water to move around more freely. Another problem could be a high water table. I live 12 miles from the nearest lake but can find ground water, even in mid summer, 12 inches down and neighbors have dug ponds simply by removing some soil.

Not knowing much about your soil or your bed, it is difficult to make specific suggestions. There are many variations on clay, not many of them very nice.

I live on a very heavy, expansive, high clay soil - USDA classification is "clay", and it's that way for more than 7 feet down. If I filled 6" deep x 14" wide hole with water, I suspect most of it would still be there after 5 hours, yet many things grow beautifully. Getting the irrigation right is a big part of the challenge. Other important factors are organic matter in the soil (more helps up to the limit of stability in the soil for perennial plantings), bed design (raised is better), and high planting in the beds. Part of the intent is not only to maximize "drainage", which is poor at best, but also to assure runoff when we have our customary Gulf coast toad-strangler T-storms that can easily drop 1-2" of rain per hour.

If everything else is growing well I would suspect your problem was plant specific or may have been, as you suggest, a reflection of a temporary irrigation issue. On my soils it is very easy to over-irrigate; the soils hold moisture extremely well. Learning to deal with that was a matter or regularly sticking a finger in the ground and probing around, which I still do. I'm not convinced that drip irrigation, which tends to deep irrigation, is a good thing on the clay soils here. They tend to hold moisture at 4" depths much, much longer than nearer the surface.