The reason why your frozen vegetables go soggy is because the freezing process of the vegetables damages the vegetables cell structure and causes water to be released from the vegetables when they thaw out.
The water along with the inherent moisture of the vegetables can cause a limp and soggy texture of the vegetables when they are cooked.
When the water in the vegetables freezes it expands and causes ice crystals to form on the vegetables which can damage the cell walls of the vegetables and cause them to rupture.
As the ruptured cell walls of the vegetables release water when the vegetables thaw out it leads to a soggy and watery texture of the vegetables.
To prevent your frozen vegetables from becoming soggy you should blanch the vegetables before freezing them.
Blanching the vegetables before freezing is when you get a pot of water boiling and then you place the vegetables into the boiling water for 1 to 2 minutes and then immediately after removing the vegetables from the boiling water you then place the vegetables into an ice bath to stop the cooking process.
Store bought frozen vegetables are blanched before freezing to prevent them from becoming soggy.
If your frozen vegetables are store bought and they still go soggy or if you have blanched your vegetables before freezing and they still go soggy then they can sometimes go soggy due to the boiling or steaming.
The water from boiling or steaming the frozen vegetables can further exacerbate the sogginess of the vegetables due to the added water.
Frozen vegetables may also go soggy if you thaw them out before cooking.
You can cook the frozen vegetables straight from frozen which can also help prevent the frozen vegetables from becoming soggy.
Use high heat when cooking your frozen vegetables which quickly cooks them and prevents the vegetables from becoming soggy.
You can also roast or air fry the frozen vegetables which helps the frozen vegetables to crisp up instead of becoming soggy.
And if you do thaw your frozen vegetables you should drain any excess water from the frozen vegetables and pat the frozen vegetables dry before cooking them.
And use a very hot pan and stir fry the frozen vegetables quickly to prevent them from becoming soggy.