I have 3 x 33g tanks with those dimensions. All three are bowed. Never causes any problems except my lid doesn't sit squarely.
They are not made anymore with those dimensions. The next tank I bought is a 40 gallon tall (same footprint but 20" high). This one has a center brace. For some reason, they didn't brace the 18" tall tanks and they all bow. It seems crazy a little strip of plastic protects against that....but I guess it does.
My 37g (2ft x 1ft footprint and 24" high) had a center brace, but not the 18" tanks.