Stringy Rhubarb

Rhubarb does have a lot of strings naturally. But as your plants get older, you may find that the stalks are thinner and stringier. This is a sign the plant needs to be split into smaller plants and given more room. You need to do this every two or three years or so.

Also, rhubarb is a heavy feeder. I surround mine with 2 or 3 inches of organic compost in the spring when it first sprouts and it feeds it all summer long.

I believe that is the nature of the stem.

Maybe you're letting it get to old before you cut it.