You might find some clues here

https://gardenerspath.com/plants/vegetables/bitter-lettuce/

Depending on where in Mi you are, your biggest factor right now might be the above average temps and how dry it's been. It tends to happen to a lot of lettuce during the hot of summer, but it's come on a bit early this year for a lot of the state. It tends to trigger bolting, and that can bring on the bitter. Some of this can be tempered with even watering and consistant moisture in the soil. If you aren't currently mulching, you should do so. A bit of a shadecloth to hold off the harshest part of day can sometimes help. But some of it is just out of the control of the gardener.

Some varieties are more tolerant of summer coming on. Some get bitter quicker. So what kinds of lettuce you have might be a factor. If you are growing a mix or multiple varieties of lettuce and all of them are getting bitter, it's more likely envoronmental than variety specific.

Age can be a factor too, the older they are the more likely to go bitter with many varieties. Try picking leaves when they are smaller and younger to see if they have less bitter. If you are already using the younger leaves and they are already bitter, more likely to be environmental factors.

There could be nutrient lack, specific nitrogen. If you have good plant and leaf development, it's likely not an issue. If you have stunted or poor leaf development, feed your soil.

There's aster yellows too, that causes lack of color and deformed plants/leaves- but you didn't say your plants are like that.

You can try soaking your lettuce in cold water for 15+ minutes to help "soak out" the bitter and refresh it with water. Some folks like to add baking soda to the water or to put it in the fridge to help with the bitter.

I'm in Ohio and I always find lettuce to get bitter in June, getting worse and worse as the days get hotter and longer. I always grow transplants and try to eat most in May and give up on it by the middle of June. Soaking in water in the fridge seems to help.

I'm not sure how you grow them, but try to get them growing early to pick before the heat.

you did not list the variety of lettuce you are growing?

I have a wide variety of leaf lettuces. These are not ones that are identified as heat-tolerant, since I started them so early in the spring. We are having an unusually warm late May - June, so that is probably part of the issue.

It is bolting, it does not taste good, I am in Ohio too, now is aobut the limit. Planing so to grow behind tomatoes extends harvest time a little, shade. As soon as you see it start to get really tall REALLY fast it is bolting, harvest asap

Pics from today

'Flashy Trout Back' is very good soft buttery chartreuse romaine, about to flower. Actually the lower leaves have still been edible in June. It's been a cool late Spring except for one week.

Below 'Magenta' is a slow bolting crisp type. Doesn't taste too bad, but never really had much taste to begin with, thick leaves.

'Red Sails' is a very vigorous grower, close to flowering at around Solstice

Radicchio looks good, too attractive to eat, might sell at a stand though I'm sure it has a strong taste. First time I've not had a mammal ruin them.

Lately I've been testing the theory that any time the lettuce soil gets dry on a warm day, it produces bitterness that then stays around for some time. So I've been watering the heck out of it. In addition to my trickle irrigation, I hose-water each day when it hasn't rained recently. The trickle irrigation waters deep into the soil, but lettuce is shallow-rooted so I douse it w/ my hose. (Besides, I love to water in the morning--just me, my garden, the birds, the fresh air...)

So far it is working! My lettuce has been tasting great!